Look up the "sunk cost fallacy" - don't stay where you are because of what you've invested into it. Take the best option looking forward.
As others have said - 5/4 is great and you may well miss it. But as you are young (and without family yet?) then joining the BA MSL now is a good choice, as you can do your most junior years while you are more flexible.
Long Haul is something you're never going to get with FR. Nor is crew catering, aircraft variety, night stops etc. But I understand FR pays much more than BA, certainly in the first decade which is where it matters most.
The work of another type rating wouldn't bother me... but I would personally like to get 1000hrs in any type to feel like I understand it. Having said that, if you are just applying now, it could well be many months to go through the selection process and get your EF start date, so you may feel like you've done your time by then.
good luck with your decision. For reference, I joined BA (straight onto LH) just when people would have told me not to bother - I was a very well paid SH captain, ~40yo, wife+kids to support. But LH had always been my dream, and though a big drop in pay, i still live the same lifestyle as I've always been pretty frugal 😆 I'm very happy with my decision - and if I wasn't then I'd reverse it.