The problem lies more with the phone itself no longer being supported, as both Calyx and Graphene only do harm-reduction updates after end of life, not full security updates. You will be taking a risk using either, but both are better than stock android.
For some reason you'll find a lot of Calyx/non-graphene os hate on lemmy (just look at the dowvotes on anything calyx related, even on this post). But if your threat model is just combatting coprorate data harvesting, de-googling, or further securing your phone, it works well and does as promised.
You should also look into Fairphones with Calyx. They're a bit pricey, but they get hardware support for 10 years instead of 5 (most android phones) and they are built with replacable parts in mind to prevent e-waste and unnecessary cost.
So in other words, yes you will have to buy a phone every 5 years (or 10 with fairphone) in order to have comprehensive security, even with graphene or calyx.