Back in the day (2010-11) I used to work at a place which had Bru Server and a tape library. When the library stopped working, they switched to Bru PE and a single tape drive.
I still have a working Bru system and a compatible LTO-6 and LTO-8 drives to be able to restore the occasional Bru tape.
In your case, since you think the LTO drive may be the issue, if you can borrow another LTO drive and have the necessary interface to connect to it, then you could test and see if it works.
My advice is, if you have a working Bru installation, then don’t bother with Argest. Bru will work just fine for tape restores.
For your 488 Bru tapes, you may want to, in the long run, migrate them to LTFS and LTO-8.