Well you don’t but the bicycle chain is one of the most efficient things humans have ever invented
And if you do stuff to make your power lower no one really cares. Mostly indoors your chain isn’t getting dirty and a dirty drive chain is wearing stuff out more than drastically making it less efficient. Even lubrication doesn’t vastly affect the efficiency. I mean maybe Bradley Wiggins for his hour record paid a lot of money on special chains and waxing to get 3w or something but there’s a lot of people kidding themselves performing chain cleaning rituals.
Mostly all anyone else cares about is if you’re racing and your power is higher than it should be - and even then it’s not every race or event. If you can coat your chain in dirt and make it more efficient then perhaps they’d start asking for photos of clean chains next to a copy of today’s newspaper.
It’s your money but it really makes little sense imo to buy power meter pedals to measure how accurate a £400 spin bike is. The pedals would cost more.
Not the least because the way you suggested you would have to spend £3000 on a smart bike implies you don’t have a bicycle? If you don’t have a bicycle why buy power meter pedals?
If you had a bicycle you could put that on a trainer that costs £250-600 depending on make, model and features. That would probably be a better fit with zwift than the fr30.
If you don’t have a bike the options are more expensive for zwift perhaps than spin bikes, but zwift ride + kickr core is £1200 - albeit ties you into zwift platform.
There are a couple of expensive options like the tacx neo smart bike or wattbike atom around £2k and above that you’re just throwing money away imo.
But, there are not many use cases on zwift where the accuracy of your power matters that much. It’s not going to have a huge impact on how fit you get. In fact power meters haven’t really been a thing for that long, especially not as affordable things that keen cyclists had.
What is more important in buying decision is how much time and money you want to invest into indoor cycling vs outdoor and that perhaps is the best way to think about what to purchase for zwift.
If I didn’t have a garage with 2 road bikes in it already, I would have got a wattbike atom. As it is I got a kickr core and put my road bike on it. The road bike has a power meter but I haven’t even bothered putting a battery in it to see if they match because loads of other people already have.
But if I’d had a spin bike that connected to zwift I would have probably used that for a while - just like I had a wahoo kickr snap that I used for years and before that (and before zwift) I had a dumb trainer that I used for years. None of these things has affected how fit I got. In fact I was far more fit when I rode the dumb trainer and had a hybrid than I am now with a kickr core and a di2 carbon bike - albeit it was perhaps slightly less easy to measure that because power meters weren’t a thing. Might could just flash up ‘you suck’ these days.
But for most other things the main thing is consistency in riding will get you stronger on the bike even if the power meter power can’t be completely trusted to be accurate.