Someone being brought in at 65m won't want to sit on the bench. He will be a first teamer, I'm sure Rodri could be converted into a CB given training. He's seems another Javi Martinez from what I've seen.
Yes, I would be confident in Eric Garcia. Kids make mistakes but so can older senior pro's. He's never really put a foot wrong in the games I have seen him and looks as assured as a young Centre Back can considering his age. Sandler also needs games and time to show his worth, he obviously has potential and seems to be a very intelligent lad with an eye for a pass. We will never know their true potential unless we put the trust into these players. It's exactly what Pep did with Kimmich and Zinchenko, Klopp did with Trent and Gomez and Fergie did with Scholes, Beckham and Neville.