iPhone SE – iphone, itunes, Mojave, backups, and two questions.

I back up wired via itunes (by the way, maybe it’s more of an itunes topic?)
so question 1: can we safely delete the backups preceding the last system update (in library/itunes/iphone) because casually I just took a look, it eats up 50 GB.
And question 2: I may ask to save on the mac, systematically when I come back it is icloud which is checked. Except the backup takes 6GB and I’m not going to pay apple for the sake of backing up to them. How do I keep the tick on ‘this computer’? If it’s possible…

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