Hey hey,
We did the fiance visa, it is quite a bit more expensive and you only have a maximum of 6 monthes to spend together and then you have to be married and applied for FLR at the end of it. I’m happy we did it, but we could have saved a lot of money just getting married and skipping that step.
It doesn't make it any easier to do anything. It just gives you a few monthes to live together before hand. He won't be able to work at all (even if it's for a US company),look for a job, volunteer, or do anything remotely like work. He'll need health insurance for those 6 months as he won't be covered by the NHS until his FLR is approved after you're married.
If he could afford to take a sabbatical from work he could come over for a few months beforehand on a visitor visa, just realise he would have to go back to the US to apply for a settlement visa. You also aren't able to get married in the UK on a visitor visa. Gotta love it. 🙄