I setup key 3 as middle key, and emulate scroll wheel by holding it and moving the trackball.
But this setup puts stress to my thumb, so I look for trackball with hardware scroll.
Logitech M570 Wireless Trackball was not considered for being wireless and thumb-operated.

SetPoint – Logitech Support + Download
uberOptions: Enable all options on all buttons in SetPoint - uberOptions

Linux setting

Logitech_Marblemouse_USB - Community Help Wiki
Logitech Marble Mouse - ArchWiki

Put this file in locate xorg.conf.d | grep d$:

# - - - Logitech Marble Mouse Settings - - -
#
# For the sake of comments below, a Logitech Marble Mouse has buttons
# labeled from left to right: A (large), B, C, D (large).

# Preferred options for right-handed usage are:
# Left to right: A=1,normal click B=2,middle-click C=2,middle-click D=3,right-click
# Press button B (hold button while rolling trackball) to emulate wheel-scrolling.

# Preferred options for left-handed usage (saying 'alternate-click' instead of 'right click'):
# Left to right: A=3,alternate-click B=2,middle-click C=2,middle-click D=1,normal click
# Press button C (hold button while rolling trackball) to emulate wheel-scrolling.

# The trackball can scroll in two-axes, unlike a typical wheel mouse. Adjust the
# settings to constrain the scroll action to vertical-axis-only if you prefer.

# Pressing both large buttons simultaneously (b) produces a "back" action (=8). Finally,
# pressing and holding button B while rolling the trackball emulates wheel-rolling action.

Section "InputClass"
    Identifier "Marble Mouse"
    MatchProduct "Logitech USB Trackball"
    MatchIsPointer "on"
    MatchDevicePath "/dev/input/event*"
    Driver "evdev"

    # Physical button #s: A b D - - - - B C b = A & D simultaneously; - = no button
    Option "ButtonMapping" "1 8 3 4 5 6 7 2 2"
    # Option "ButtonMapping" "1 8 3 4 5 6 7 2 2" # For right-hand placement
    # Option "ButtonMapping" "3 8 1 4 5 6 7 2 2" # For left-hand placement

    # EmulateWheel refers to emulating a mouse wheel using Marble Mouse trackball.
    Option "EmulateWheel" "true"
    # Option "EmulateWheelButton" "8" # Factory default; use "9" for left-side placement.
    Option "EmulateWheelButton" "8"
    # Option "ZAxisMapping" "4 5"
    Option "ZAxisMapping" "4 5"
    # Option "XAxisMapping" "6 7" # Disable this for vertical-only scrolling.
    Option "XAxisMapping" "6 7"
    # Emulate3Buttons refers to the act of pressing buttons A and D
    # simultaneously to emulate a middle-click or wheel click.
    Option "Emulate3Buttons" "true"
EndSection

Windows setting

Scroll modifier for mouse/trackball in windows - Super User
~~Marble Mouse Scroll Wheel~~ (dead), home for marbleinst.exe
MarbleScroll for Logitech Trackman Marble – Primož's Few Projects
X-Mouse Button Control

fn1. with little practice, one can actually use any pointing device with both hands

Adesso iMouse T1

I got this thingy from Taobao. Turns out it copied the design of Sanwa MA-TB41.

It has a scroll wheel on the left hand side of the device. It wasn't bulked outwards enough so it cannot be reached by the thumb when you are using the trackball naturally. And pressing middle button is difficult and quite often trigger scroll.

Pros:

cheap, < HK$250
finger-operated
with scroll wheel

Cons:

scroll wheel is hard to reach, thumb movement needed
scroll wheel is too close to the trackball, usually have to bent thumb to reach the wheel
pressing middle button will trigger scroll, and vice versa

Linux setting

xinput --get-feedbacks "Trackball Mouse"
xinput --list-props "Trackball Mouse"

xinput --set-ptr-feedback "Trackball Mouse" 10 5 10
# Evdev Wheel Emulation Inertia (285):    30

# for 800dpi Adesso iMouse T1 trackball

Section "InputClass"
    Identifier "Trackball Mouse"
    MatchUSBID "195d:1009"
    MatchIsPointer "yes"

# set the following to 1 1 0 respectively to disable acceleration.
# trigger acceleration of 2 after 5 units
    Option "AccelerationThreshold" "7"
    Option "AccelerationNumerator" "1"
    Option "AccelerationDenominator" "2"

# some curved deceleration
    Option "AdaptiveDeceleration" "1"
# linear deceleration (mouse speed reduction)
    Option "ConstantDeceleration" "1"
EndSection

Kensington SlimBlade K72327

SlimBlade™ Trackball | Trackballs | Trackball Mouse | Wireless Mouse - Kensington | Kensington
Then I turned to the one in consumer market who still innovates in trackballs: Kensington.

Slimblade Trackball
Hands-on: the Kensington Slimblade trackball | Ars Technica

TrackballWorks | Trackball Customization | Kensington

Pros:

dual laser sensors
hence can scroll by twisting the trackball
finger-operated

Cons:

expensive, official price at HK$1000, ~HK$600 at Taobao

Linux setting

kensington slimblade trackball linux - Google Search
Living and Programming - YJPark's Blog: Using Trackball on Linux
Kensington SlimBlade Trackball 軌跡球在 Linux 中的使用設定 - 石頭閒語

# enable wheel emulation
xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation" 8 1

# use button 8 (upper right button) as the emulation button
xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Button" 8 8

# use for vertical and horizontal scrolling
xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Axes" 8 6, 7, 4, 5

xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Inertia" 16 15
xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Timeout" 16 300

# button 8 don't work as "back"
xinput set-button-map "Kensington Kensington Slimblade Trackball" 1 2 3 4 5 6 7 10 9 8 11 12

# wheel emulation by toggling, not holding
# requires driver patch
# xinput set-int-prop "Kensington Kensington Slimblade Trackball" "Evdev Wheel Emulation Button Toggle" 8 1

WIP

Section "InputClass"
        Identifier "evdev pointer catchall"
        MatchIsPointer "on"
        MatchDevicePath "/dev/input/event*"
        Driver "evdev"
        Option      "EmulateWheel"      "true"
        Option      "EmulateWheelButton"    "2"
        Option      "XAxisMapping"      "6 7"
        Option      "YAxisMapping"      "4 5"
EndSection

Elecom HUGE M-HT1DRBK

Elecom HUGE is now my personal favorite

Pros:

cheaper then SlimBlade, ¥370 (¥299 at 1111) at Taobao
physical scroll
soft pad as wrist rest
many programmable buttons
wireless

Cons:

right hand only
wireless (needs AAA batteries)

Wireless track ball (forefinger, middle finger operation type) - M-HT1DRBK
ELECOM M-HT1DRBK Wireless Trackball Mouse Review - Nerd Techy
Elecom HUGE Trackball
Elecom Mouse Profiler – Troubleshooting Guide - OxGadgets

Elecom Huge = Gaming TRACKBALL? Bearing replacement / fix / swap - YouTube screws location

Linux setting

Linux users with Elecom Huge how have you binded your fn keys? : Trackballs
Elecom Huge Trackball xorg configuration : Trackballs
First trackball - Elecom Huge - Any Linux Users? : Trackballs
Elecom Mouse Profiler – Troubleshooting Guide - OxGadgets

L-Trac

L-Trac Trackball Review
Slimblade vs L-Trac Trackball

三和

三和轨迹球_淘宝搜索
 価格.com - タイプ:トラックボールサンワサプライ(SANWA SUPPLY)のマウス
 日本 SANWA 有/无线轨迹球鼠标大鼠制图绘图专业鼠标-tmall.com 天猫 ¥299

Honorable Mentions

优鼠手握式轨迹球

Kensington

Orbit Scroll Ring was not considered for having only two buttons. Middle button is crucial for me.
Orbit Wireless Mobile was not considered for being too small and wireless, but that may be a reason for you to choose it.
The tilting angle for Expert Mouse is too high. It seems that it would cause wrist pain for prolonged usage.

Elecom

トラックボール - ELECOM | トラックボール

Deft series is so-called index finger trackball.
Deft Pro is smaller then Huge, seems to have better built quality.
Deft is even smaller, but good for one hand off-the-table use.

Elecom DEFT Pro Trackball
Elecom DEFT Trackball

Trackball

TouchPad

Window settings

Linux X settings

Button Mapping

scroll wheel

smooth scrolling

acceleration

Products

Logitech Trackman Marble

Linux setting

Windows setting

Adesso iMouse T1

Linux setting

Kensington SlimBlade K72327

Linux setting

Elecom HUGE M-HT1DRBK

Linux setting

L-Trac

三和

Honorable Mentions

Kensington

Elecom