Zwift can be done on a dumb trainer, but it will be just as boring as any of the old cycling training videos out there because its one-note for resistance and you have to force yourself to shift to make it anything else. The real magic is when a smart trainer adjusts resistance.
I rode a Kickr Snap for 4 years without issues until I upgraded recently to the Kickr Core. You don’t have to buy a new cassette for a trainer if you have a bike you plan to dedicate to the trainer anyway - just take that cassette.
I will say that while I loved the Kickr Snap - the direct drive is a much better ride. I did have wheel slippage, especially on really steep inclines (~15% percent). My husband also had a snap at first, but his wheel slippage was much worse (puts out much more power) so he upgraded to direct drive much sooner.
With race fees saved, get the new trainer. Have you looked at DCRainmaker’s reviews?