You might try this one, I don’t know the name for it or anything. But instead of trying to trap their arm on your chest where they can clearly see an armbar coming a mile away, push their hand kinda down to your opposite “pocket” so their arm is kinda under them across your belt. Then either hook under their free arm or double grab their lapel and back of collar, basically anything to keep them postured down so their arm doesn’t pop out. Then throw the leg over for the armbar and finish it low. It’s super fucking tight and a little bit more tricky that they might not expect it, at least the first time lol. I’ve had success with it.

You can also start with the reverse kimura grip , it’s basically a similar angle, and instead of going toward a baratoplata or bicep crush just go for a regular ambar. They will probably posture on you at first but the reverse kimura keeps their arm close, just don’t lose it when you switch grips for the arm bar.

I’m no expert grappler lol but I have hit a lot of in the guard armbars with tricky setups. Especially in Gi where it’s super hard to yank the arm out. In Nogi, at least for me, I feel like them being on top and the slipperiness makes it way easier for them to just start the lawnmower and yank their arm out. I’ve had way more success nogi with getting them off the back, either straight on it or from a back triangle.