The only restrictions on rollbacks are to pre-GDPR versions, which aren't available via Steam unless you enter a code that you can find on the Paradox forums. I think it's a legal thing, because the old versions aren't GDPR compliant they must be opt-in. But as a new player, you'd have no need for them.
I have mixed feelings on Paradox DLC. They certainly made it too fine-grained for a while there, though they've stepped back on that front. But also, EU4 is a ten-year-old game. If Civ3 were still being updated in 2011, ten years after release, it would likely have had ten expansion packs, and as the retail price was $30 (at least where I lived in the U.S.), the DLCs would cost $300 at full price - plus $50 for the base game, for $350 total. There's more sticker shock when you see the total price for all EU4 expansions on Steam, ten years in, but IMO it winds up being in the same ballpark as what Civ III could have been with ten years of updates. For that matter, Civ VI with all expansions adds up to a decent sum too, although Firaxis is more generous with the discounts than Paradox (who mainly only does > 50% discounts on DLCs via the rare Humble Bundle).
If you liked the original EU, you'll probably like EU4. I only went as far back as EU2, but the general spirit is there, though obviously a lot has changed. As for the learning curve - well, you've played HOI4 and Stellaris, it's similar. Halfway decent at explaining things, and the Wiki's always there for what needs more clarification.
Although, as a moderator of the Civ3 forum, I should warn you that if EU4 is to your liking, it may decrease the amount of time you spend playing Civ. Thus I'd carefully consider whether that's an outcome that you want to open the door to before buying it. Going back to Civ3 in never a bad decision.
