I feel like you answered your own question in your first sentence quoted above. The volume has been decreased, so you’re assuming you need to add something in (like more assistance work than you were previously doing). But, the volume decreased because intensity increased. So don’t compensate for the lack of supplemental volume with more assistance work, because that lack of supplemental volume has already been compensated for with higher intensity supplemental work.
To put it a different way, you have a “training bucket”. You felt that bucket was full when you were doing this:
Bench 5/3/1
Bench BBB 5x10 50%
Lat work 5x10
Triceps 3x10
But now that you’ll be doing this…
Bench 5/3/1
Bench BBB 5x3 90%
Lat work 5x10
Triceps 3x10
…you feel you need to change/add something in, like extra assistance work, to fill that training bucket back up. But that bucket is still just as full because the volume was replaced with intensity.
I’ve recently done the BBB progression from the Beyond book. The way I looked at it was: You put some muscle on with the high volume work, then the focus shifts towards getting stronger, therefore volume goes down and intensity goes up. Adding in a bunch of assistance in order to maintain the volume will get in the way of the getting stronger part.
TL;DR: Don’t add more assistance.