^There certainly isn’t an issue doing that as long as there is clearance (when the suspension compresses).
To save some money, you could consider keeping the 14” spare if you go this route.
There is no inherent issue with a smaller diameter tire in your situation, as a spare.
Those wheels go for about $60 a piece, online, as a comparison.
If you’re changing out rims too, check etrailer.com for cost comparison. They sell mounted tire and wheel sets free shipping.
Regarding keeping the old tires another year really depends on how they were kept and how hard/long you plan on running them this year.
I run my snowmobile trailer tires much longer generally. But they generally don’t go long distances, aren’t loaded to capacity and it’s cold out!
Boat trailer, 1 fender costs as much as 2 sets of tires if a tire shreds it, boat gets tugged at 80mph for hours on end in hot weather. I replace them every 4-5 years without question.