I understand what u saying bro... but hear me out...
Sasuke can be base form or maybe not since Hagoromo gave him chakras aswell but nvm that...
Sasuke base from - would be deactivating Rinnegan but thats impossible - If he could do that, that^ would be base Sasuke
Naruto's case... is troublesome since... we alrdy talked about it - Kurama + 9 bijuu chakra is alrdy mixed with his chakra- Ofc he can increase his power lvl but Naruto was never base... ever since the beginning
Let me explain a bit.. I hope it will answer your questions
Sasuke cannot deactivate his Rinnegan correct??
but he can deactivate his Sharingan right??
Good so the weakest Sasuke can get is using just his Alway-on-Rinnegan alone(that's with Sharingan deactivated)
This becomes BASE Sasuke because he can't get weaker than that.
Sasuke can now power up by activating his Sharingan to 3T or MS or EMS.. In any of these cases he'll no longer be in BASE because he moves from his weakest form
Now Naruto has Kurama chakra mixed with his.
So in his weakest form, Naruto has Bijuu chakra flowing in him. That might increase his durability but it's still his weakest form hence BASE
Naruto can also power up by going SM or BM or BSM or RSM without cloak or RSM with cloak. In any of these case Naruto would no longer be in base anymore because he powers up to another level which isn't his weakest