I like to use the backfill feature to prevent mobs and more following me in the tunnels I make using the bot but also to prevent players from following the paths that are blatantly used by a bot so maybe improving it so that you can choose what blocks can or can't be used and what ones should get priority because I was mining at deepslate level and it was filling with cobblestone and when I ran out it stopped filling so it would be nice being able to allow it to fill with deepslate too. Next suggestion is maybe not part of #backfill but making it place torches every so often like maybe when strip-mining instead of filling with blocks and also I noticed that sometimes it fills behind it self then mines it again to get an ore behind it which is kind of useless to place a block that you'll mine the next second and it also uses durability on the pickaxe even though that's not a problem for everyone.

Backfill uses whatever is listed in acceptableThrowawayItems so you could e.g. use #set acceptableThrowawayItems deepslate to force it to use deepslate.