ignoring n+1, would a small 24" dirt/street bike help improve skills, jumping and tricks?
i'm thinking something 2nd hand around 400-500€ should more than suffice and it is a little more versatile than a BMX (my vertically challenged sister could ride it when she is over to visit, for example).
I thought the same and got a BMX.
I found the twitchy geometry and lack of inertia in the wheels made me ride compeltley differently, when I went back to mountianbikes I found that I rode differently as I was now more active in making corrections on the bike rather than relying on the slack angles and big wheels to see me through, which ultimately makes me faster.
Not sure a 24" jump bike would be sufficiently different to a LTHT unless you just want another bike for jumping to save the nice bike from being broken doing something silly. I know someone who keeps his old 222 for this reason despite saying his new 5 is faster downhill he want's the 5 to still be working the next day.
should i get a small 2nd hand dirt bike?
Have a GT Chucker for exactly this purpose.
It has other duties as a bad weather MTB hack and town bike but otherwise it really is invaluable.
Cheap enough not to care about and tough enough to take the knocks but still good enough to have some fun with.