[2023] Made new game, played a good while, saved... came back days later and CANT LOAD because it says I dont own DLC or some crap.

What DLCs do you have? Which one is the save saying it needs to be able to load?  

That's odd, because Horizon Signal is not a DLC, it's an event chain that's part of the base game

https://stellaris.paradoxwikis.com/Horizon_Signal
https://stellaris.paradoxwikis.com/Guardians#Horizon_Signal
Can we see a screenshot of that error please? Is it just affecting one particular galaxy, or any one you play?