Firstly a Contract is a Meeting of Minds, the forty pages of small type in a PDF are nice, but it's laughable that you pretend you thought everybody read those before using your free service. And if they didn't read them, they clearly cannot agree with just every random term you threw in there and so it can't all be part of that meeting of minds, so there is not, in fact, a contract with people with those terms.
OK, so what _was_ agreed? Well, a court is going to decide what a _reasonable_ person thought they were getting into, and they'll use legislation (such as that from the GDPR) to help decide that. They'll also keep in mind a theory about relative power. You wrote these T&Cs, so the court is going to conclude that you should have taken that opportunity to add any terms you really cared about. On the other hand the _user_ wasn't able to edit the terms, so really anything they reasonably expected should probably be acceptable.
The GDPR says that you need to have the user explicitly opt in, they get to reasonably assume that's how it works, you can't change that in the text they didn't read.
You might think, "Aha, but I made them check a box saying they agree they read it". Too bad, that doesn't help for a very simple and pragmatic reason:
Judges are people too. When you explain this theory to a judge, who like other people has had to check loads of these stupid "I agree I have read a 400 page document before using this free service" boxes, they are going to look at you like you just said you think they're an idiot.
If you're thinking maybe you can try this on and see for yourself, you'll probably have to be your own lawyer. Certainly in the UK no competent lawyer will take that work. Years ago the UK passed a law banning certain contract terms in "short" residential leases (a "short" lease would be e.g. renting a house for a year). Immediately scumbag landlords wrote new contracts that said basically "I, the under-signed, agree to these terms even though they're not allowed" and then demanded their tenants sign the revised contract instead. Judges were not happy, and I pity the fool who first appeared in front of a judge trying to argue that this was somehow legal when it's obviously not.
If you're so sure your users want to explicitly agree to let you do this, make it a separate opt-in, like the regulation says. When, to your disappointment, they don't want to, that is a _learning opportunity_ for you. Take it.