Caravan mobile internet - which data SIM?

Hello

I've been a Smarty mobile user for some time, and have recently bought a caravan with mobile internet.

Please can someone help me decide which type of SIM to get? Some months it will get little use, others perhaps a couple of zoom meetings up to 90 mins and if bad weather the odd movie or TV catchup. So my useage could vary from very little to quite a lot. Do Smarty add ons expire? I see there are data passes which might be the way to go. I also saw on here a post about using existing phone SIM, though the router is in the most awkward place to get at. Any help to navigate this so I can make a sensible decision would be very welcome.