they could do a film set in the past. doom and ff mess up and get trapped in another dimension. wandavision and/or the snap release them. that gives you 1 or two films without treading on modern day MCU's toes.
set the original in the 60's or 70's. delve into shield, yancy street, space race, quantum being new science, major computing power coming online. focus on the mental struggle between reed and doom. the class struggle of ben. the invisible girl to invisible woman (**ahem** invisible PAWG) growth. Johnny can be the frustrated dreamer who is along for the ride. they could even have an entire story in the quantum realm or microverse, or stuck in a dimension which is in the future.
then they buck up on kang, in FF style lose and have to run / escape back to MCU current time. mix in a bit of skrulls, kree and "TFATF mi familia" along with charismatic actors and it might work.
like how captain america, captain marvel and the GOTG (along with DC WW) got breathing room by setting their introduction "out of time/space".
you can squeeze out 2 FF films without messing with current MCU too much .. and that covers quite a few years.