All Steam achievements are available for any new characters you create on Steam. Yet, it is not sure that Steam will register those achievements consistently. Some Steam achievements may still be acquired on the same character if you complete certain tasks.

If you transfer an Epic save files to your Steam account (Steam supports Borderlands 3 later than Epic), the save data won’t sync between these two platforms, and you won’t receive your STEAM achievements that you already have with that save files. However, the files transferring process will only update the locally saved files, for cloud existing savings, just refresh them to reflect the new files. Then, when next time you start your game, all preferences and characters in both Epic and Steam version of Borderlands will show up. Those files include a profile.sav (tracking your Options preferences) and all numbered.sav files (representing your Vault Hunters). To transfer files from Borderlands Steam save location to Borderlands Epic save location, just copy all items from the source platform folder, and paste them into the destination platform folder.

Borderlands 2 level 50 save