Sorry I missed this topic before.
Sounds like a lovely trip, I think good to avoid Porec as a place to stay, it is small and very touristy. Great for a visit (the basilica is lovely) but not ideal to stay in. Pula is well worth a visit for the Roman arena and my parents always tell me things about Rovinj and Koper.
For logistical reasons I stayed in Rijeka, it is pleasnat but if you don't visit no great loss.
When we hired a car in Croatia (in 2018) we had to pay an additional fee (around 30-40 I think) to enable us to drive it into other countries (specifically Bosnia), some internet forums said you could avoid this, but without it Sixt wouldn't give us the Blue book documents that were required at the border crossings. Croatia isn't in the Schengen area so there are still border posts when crossing to Slovenia, so will probably have the same issues there. Slovenia > Italy was completely free movement though.
Also if driving into Slovenia, you will need a
toll vignette to drive on their highways, the shortest period you can get this for is 7 days.
I don't know how much time you wanted to spend driving, but Aquiliea and Palmanova (Venetian defences) are both close by if you wanted to venture into Italy. Also Cividale del Friuli can be combined easily with Idrija if you wanted to pick up the Longaboards site as well. I would thoroughly recommend all three Italian sites, but understand not everyone wants to maximise their WHS ticks at the expense of relaxing a little bit.