You can get stock firmware from
https://www.sammobile.com/firmwares/ - enter the model number (SM-T350) and select your country, and it'll list the available packages. For the T350 there should be both Nougat and Marshmallow packages. If you haven't downloaded from sammobile before, you'll need to register an account.
The version of TWRP that you should use is 3.5.2_9. You won't find it on this thread, which has been inactive for quite some time. See the FAQ Q5 in
@retiredtab's LOS 18.1 for the T350 thread
https://forum.xda-developers.com/t/...or-galaxy-tab-a-sm-t350-june-11-2022.4305903/.
To proceed with installing TWRP, you might need the Marshmallow bootloader. This is because of a problem between TWRP and the Nougat bootloader for T350s with a BOE screen - the Nougat bootloader doesn't turn the screen backlight on, so TWRP's screen is blank. The Marshmallow bootloader does turn on the backlight.
To check whether your T350 has a BOE screen, install the
DevCheck Hardware and System Info app from Google Play and look at the Hardware tab, Graphics section. If you have a BOE panel, the Panel item will contain "BOE". If you don't have a BOE panel, you can go with the Nougat bootloader.