What if you got an interactive trainer and used it in ERG mode with a singlespeed kit (single sprocket and spacers)?
That works but then you're so totally reliant on the turbo resistance scaling & timing of such. For example my Vortex can only change 1% resistance every second, so a 10% ramp has at least a 10 second delay before the resistance is fully applied and then another 10 seconds before it's removed. 20 seconds of resistance changing 'delay'. Meanwhile you're actually riding the 10% gradient and the 10 seconds afterwards, effectively smoothing the 'ramp' and extending it's gradient by 10 seconds of additional riding. If it's only a short period you're at a disadvantage to riders with a quick resistance change as they can kick into it at the bottom sooner and then have ~10 seconds of less resistance than you over the top. A gear change to negate the slow change (so trainer has less resistance to apply) is the usual 'workaround' riders learn with slow changing trainers like this. Single speed you wouldn't have that option.
In practice as the gradients on the Zwift ramps are not that high (more gradual) things are not as extreme/pronounced and I've actually found the 'slow' power curve of my Vortex gives me an advantage on 'rolling' gradient changes like 'The Esses' on Watopia. I can carry speed onto the ramps further, kicking when the resistance is highest (others are slowed more by their quicker resistance change) and although my resistance is applied over a longer period it's much smoother so I'm able to carry speed better. I can regularly kick 5w/kg there 3(?) times on the rollers and guys on direct drive trainers will need 7+w/kg kicks and still be losing my wheel as my speed is carried - I slingshot past them on the 2nd & 3rd ramps. Helloo breakaway/escape! Quite handily the esses come towards the end of the usual race lap and the finishing straight
The increased wear rate from using a road tyre on a turbo (if any, I never perceived much beyond that of regular road use) is best just absorbed by replacing your rear tyre perhaps a little more regularly than you would normally.
Buying a spare rim, cassette and turbo tyre is a bit of an unnecessary outlay and faff. Especially given they extent of your increased chain wear from regularly switching cassettes (dependent on mileage and presuming outdoor riding continues too).
Really depends on the wear rate as a soft tyre like the GP4000sii is well known for shredding when exposed to the heat of an aggressive on-wheel turbo session.
Personally buying a £50 new/sh wheel, a £30 105 cassette and £20 turbo tyre which will last for several years (my tacx tyre is around 3 years and 2000 Zwift miles old, maybe had 5-800 non-zwift miles & has hardly any wear). So a £100 'investment'. When your road tyres probably cost ~£40 each and you change them every year, with the trainer wear halving (?) their life, that's an ongoing £40 per year. So less than 3 year ROI.
The wear on cassette is a good mention though, you'll be very hard pressed to 'balance' the wear on both cassettes so they wear at the same rate. Regularly changing chains (before they get too worn) would negate this.
Disclaimer: I change my chains far more frequently than I should so I'm biased towards that. For me they cost <£15 each and by doing so I'm reducing the wear on my £650 powertap C1 chainrings to prolong their life. They're cheaper than tyres and cassettes!
