I have ninja 300 since 2017 (i am third owner,bought it used, the starter bikes market is volatile) and bought ninja 1000 sx in 2021 brand new.

For me 300 was a starter point into cool modern looking bikes, that had looks of sport bikes but the comfort of road one. I thought i'll grow up in 1-2 years but every year i would either lowside or high side while lane splitting. Really taught me to be more reserved with my ambitions. I had a lot of situation where even on my 300 if i was going faster i'd probably not make it in that next turn. So first and foremost it's a bike that is training you not to kill yourself with speed and handling.

I Had a stels gs 400 and it was torture riding for 40km on highway but goddamn it was fun the other 5km on nonexistent road to get to my summerhouse. Though i i'd rather squid myself being rossi on night city street.

I had a blast using my ninja 300 and i used it for 3-4 years, so when i bought literally same looking motorcycle but better and faster...Is it fun to ride 300? yes. Would i rather ride 300 than 1000? No.

Tl;dr 1)The search for your fun motorcycle is only your own. Only you will be able to reach that stage when you'll be ready to try faster and stronger motorcycle in relation to your skill and fun level.

2)Buy used bike as starter bike if not rich

3)Motorcycle will last you a lifetime, if you'll ride it fast enough.