Front end and backend admin worked with only deleting the root htaccess.
I used cut and pasted into notepad++ (I previously used copy when I was troubleshooting).
I noticed that several characters were inserted into the code in one of the sections that shouldn’t have been there. These characters were only viewable when using the Cut (not copy).
I removed invalid characters and reinserted custom code via copy and it now seems to be working. Guessing the invalid characters were the reason why the 500 was occurring, but mystified as to why they didn’t show up before when viewing the htaccess directly via ftp, BPS custom code, or BPS file editor(?) I’ll have to add to notes, but the characters were only caught using cut and not copy….
Otherwise, thanks, all seems to be working now!