muni buying and tyre size advice

So I’ve learnt to ride my cheapy 20" uni to a basic level, and thinking about buying something bigger and better. As a keen mountain biker, I’m know I’m going to want to ride off-road on a uni (indeed I’ve already done some easy off-road stuff), so it seems a muni would be a good investment. However initially I’ll probably mainly be on tarmac. So I’m thinking of buying a 26" muni and putting a slick(er) tyre on. So a few questions:

Is 20 to 26 too big a jump to make? I presume putting on a slick instead of a knobbly would help with this.
The Nimbus I’m looking at comes with a 3" tyre - seems very big to me, and most recommendations for tyres seem to be smaller - is that a good tyre, or would I be better to put on one of my stock of old 2" tyres, or maybe buy something in the 2.3"-2.5" range? Do I need something with stiffer sidewalls for a uni - all my stock tyres are lightweight XC racing ones?
Given the 3" tyre, it also has a 42mm rim - if I do go with a different tyre, is a 2" tyre too narrow for this? I presume my 1.4" slicks are too narrow, so I’d have to buy something wider (or just put on an old 2" semi-slick, which has the advantage of a bit of off-road ability, but still rolls fast).
How much of a faff is it swapping tyres back and forth - no QR on a uni? I guess a bigger 29" road uni is in my future, but that is surely too big a jump right now.

Thanks - if I’m asking FAQs, please take pity on me and point me at the relevant threads - not really sure what to ask or search for.

Go with the 26. There is not a huge difference between 24 and 26 and you get to switch tires to and from your bike.

I am not a fan of the Duro 3" tire but many are. I think I was just spoilt by having a 3" Gazzalodi. As to putting your old 2" tire on there go for it. As a general rule you don’t want the tire size designation smaller than your rim width. on a 42mm rim you could go all the way down to a 1.75" road slick without problems (though it does make rim strikes and pinch flats more likely)

Before going out and buying a new tire I would start with some of your old bike tires, then try the Duro that it comes with then decide what you like better and make you decision from there.