Question about insane difficulty and sandbox

Hello,

I really enjoy the game but I started with Difficult option, which is still not challenging enough.

I restarted the run to go with Insane as I do not mind game being super hard (I am really enjoying to fully unlock games like binding if Isaac where some challenges are straight up unfair)

But what I hate is grind, to now catch Pokemon's with good IV, and do some IV breeding (I have never done it before so it would require some researching how to do it)

So I was wondering regarding experience with sandbox mode, as I understand would speed up the process of getting good Pokémon without hindering strategy part of the battles being challenging. But I read on the official wiki that some of post game is locked out. Is it something I do not understand or is it possible to enjoy full game using sandbox mode?

Thanks for your help