It’s not about whether or not he honors them. The effectiveness of those preemptive pardons can be adjudicated by the SC. But there’s a couple of considerations on that front:

• will the SC concur that blanket pardons in advance of any charges are actually allowed?
• will Trump want to challenge that, since it would potentially kneecap any effort he makes later in that regard?

But also:

• will Trump or the SC give a shit about the blatant hypocrisy and double standard if they do strike down this sort of thing from Biden, but let Trump do it? (IMO probably not)
• if the SC gives a shit but Trump doesn’t, what’s to stop him using leverage and/or stochastic terrorism to help accelerate the conclusion of one or more justices’ tenure so he can slip someone more agreeable in?
• will the judicial branch even exist in an independent fashion in 4 years?

TL;DR: who fuckin knows at this point

Also are they even valid if they haven’t been charged with a crime.

Accepting the pardon actually requires admitting to the crime. So yes, the pardon is valid because the person has to go “yes I did this thing, and accept that I’m being pardoned for it.” You can’t selectively accept only the latter half of the pardon. And that admission and subsequent pardon means the state can’t charge them in the future, because they already admitted to it and were pardoned for it despite that admission of guilt.

But to your earlier point, there’s not much stopping Trump from trying to go after them anyways; The SCOTUS certainly won’t get in his way.

Their validity has never been constitutionally challenged. It’s not really been done much in the past, the notable example being Nixon and no one challenged that.