Are you putting aside using SHS/MTH flexi track because they don’t make compatible turnouts in the style of AM and FVM? Can AM and FVM turnouts be easily connected to SHS/MTH flexi track if you lean towards this track or is it just not worth any extra work to make it fit?
In a nutshell, yes. SHS/MTH only has the R20 turnout.
Easy? Maybe. Depends on how much effort one puts into it. HOer's have been mating code 83 and code 70 to code 100 for decades. Personally, I've never mixed rail codes regardless of what scale I was working in.
Also, are you moving away from AM track because of the need to modify the turnouts? I guess I am coming to the conclusion that the complete FVM track system suits your future needs in its entirety without having the need to modify anything, albeit the rail code is not in keeping for scale use.
My modules were the only time I've used AM track, because it's what the club decided on a standard. We had one turnout at the time and we always ran Flyer or Hirail on that loop.
My home railroad always used code 100 track. If I want to run some HiRail or Flyer, I throw some SHS track down on the floor in the man cave.
As far as "scale use," I'm becoming less fussy about such things as I get older. I try to go for an overall effect, rather than down to the smallest detail. I'm not doing nor never intend to do Proto:64. If my rail's a little high, so what? Besides, FVM track rail height seems pretty close in scale to what the mainlines are using nowadays.
As retirement looms in a few weeks time I shall be busy adjusting to a new way of life, especially as our daughter has just had her first baby mid February and I have only seen him twice with lockdown now in place. Also ‘management’ has a list of jobs that need doing before I get permission to start looking at building a permanent layout for running the Flyer. So if FVM seems the preferred route to go then I need to start planning how much I will have to purchase.