bb keeps coming loose 🙁 is loctite a good idea or longer cups a better bet?

should only be a last resort. How are you installing?

Normally I use non on b bracket

with a shimano ht2 spanner. I am not convinced of the validity of the threads on the shell to be honest

Loctite blue.Medium strength

Loctite 222

I'd be very careful with this. What bike / BB are you using?

I'd have the BB threads looked at first, or even try another BB, loctite in BB threads is a really bad idea.

Raceface diablous BB cups are 5mm longer on the threaded section to add pull out resistance on heavy landings so they may be worth a try. Bearings are not as good as shimano though. Maybe Saint BBs have the longer threads too?

Copper grease and torque set to th. Shimano figures has worked for me.

Or instead of threadlock, use a retaining fluid, apply not to the threads but the mating surface, edge of BB shell, spacers and cups. Bearing fit is the easiest retainer fluid to get hold or, though most of the thin loctite threadlockers can be used as retainer, i.e. Under the heads of fasteners or on fastener shanks, rather than directly on threads.