This post is mainly a response to people that have the same needs on reddit, I tried to invite them so please be patient & welcoming !

ღゝ◡╹)ノ♡

Just for information, I tried to lpadmin ppd file only, but had a problem with missing filter file, if someone's from debian/ubuntu based distro struggles like me (error shown by cups ui : /usr/lib/cups/filter/rastertotmtr not available: No such file or directory) just asks here and I'll give you people the right file I guess.

default install

• disclaimer
  • I didn't explain how to build your own app (as it seemed I couldn't properly do it myself)
  • maybe lpadmin will be different after -v because we have different printers/usb stuff ?
  • I'm not natural english speaker & also my brain is a total mess, sorry if reading this post is a hassle
  • maybe someday I'll do a proper documentation but until then it's here, enjoy !
• build it yourself(download ppd with link just underneath) or if on arch install it from aur : https://aur.archlinux.org/packages/epson-thermal-printer-thermal-receipt
• download ppd file https://download.ebz.epson.net/dsc/du/02/DriverDownloadInfo.do?LG2=EN&CN2=US&CTI=205&PRN=Software&OSC=LX
• add driver to cups/lpadmin : lpadmin -p TM-20-3 -v usb://EPSON/TM-T20III?serial=583741561792370000 -P ppd/tm-ba-thermal-rastertotmtr-203.ppd -E
• bonus make it your default printer : lpadmin -d TM-20-3

print !

• take not that i had specific needs with orientation & custom height/width on my .pdf files, also they were already imposed.
• if default try a print lp BAT.pdf -o media=Custom.69x65mm -o orientation-requested=5 -o TmtSpeed=4
• if NOT default try a print : lp -d TM-20-3 BAT.pdf -o media=Custom.69x65mm -o orientation-requested=5 -o TmtSpeed=4

modded installation with speed option ?

• I tried to steal speed options from another epson tm printer & write them inside my own .ppd file : https://stackoverflow.com/questions/10491282/epson-tm-t88v-formatting

*%Head speed settings.
*TmtSpeedValue 1: "13"
*TmtSpeedValue 2: "9"
*TmtSpeedValue 3: "5"
*TmtSpeedValue 4: "1"

*OpenUI *TmtSpeed/Printing Speed: PickOne
*OrderDependency: 10 AnySetup *TmtSpeed
*DefaultTmtSpeed: Auto
*TmtSpeed Auto/Use the printer settings : ""
*TmtSpeed 1/1st (Fast): ""
*TmtSpeed 2/2nd: ""
*TmtSpeed 3/3rd: ""
*TmtSpeed 4/4th (Slow): ""
*CloseUI: *TmtSpeed

• failed attempd ? lpadmin -p TM-20-3-speed-mod -v usb://EPSON/TM-T20III?serial=583741561792370000 -P speed-mod-tm-ba-thermal-rastertotmtr-203.ppd -E
• lpoptions shows it all but -o TmtSpeed=4 won't be read by my printer ╰(　´◔　ω　◔ `)╯

lpoptions -p TM-20-3-speed-mod -l
PageSize/Media Size: *RP80x200 RP80x2000 RP58x200 RP58x2000 Custom.WIDTHxHEIGHT
Resolution/Resolution: *203x203dpi
TmtSpeed/Printing Speed: *Auto 1 2 3 4
TmxPaperReduction/Paper Reduction: Off Top *Bottom Both
TmxBuzzerAndDrawer/Buzzer/ Cash Drawer: *NotUsed InternalBuzzer ExternalBuzzer OpenDrawer1 OpenDrawer2
TmxPaperCut/Paper Cut: *NoCut CutPerJob CutPerPage

• other people talking about thermal printers: https://stackoverflow.com/questions/12448803/print-a-postscript-document-with-cups-and-a-thermal-printer

EDIT : adding disclaimer, changing for more clear title