DownwardDog
Riding a Brompton with a power meter.
Everyone I've ever known who knows lots about bikes has always said the same thing about conversion kits: don't do it.
Can you access the cycle to work scheme?
edit: unless you are DownwardDog. I bet he could make an e-bike conversion work really well, reliable and as long lasting as a built-for-it e-bike. But it'd cost twice as much as just buying an e-bike and require some specialist tools that need half a room and cost at least as much as a tour-de-france level e-bike (if such bikes exist).
I've made a few; I think I first made one for my wife in about 2008. Your observations on the process are correct after about 2015. Before then homebrew conversions were cheaper and better than the OEM models. Now there is absolutely no reason to make one other than the limited satisfaction to be had from the project itself.