Yes very good choice! 
Yes, the MT-6000 is also an S-tier recommendation on OpenWrt’s own forum too (just like the Banana Pi BPI-R4).
I just bought one myself, didn’t get the time to play with it yet.
But people do not complain or talk about it after some time because it mostly works just fine. 
Should be able to do just that yes. 
No reason for it to break, especially if it only keeps doing its router things.
Then it mostly comes down to your personal config and other packages you do have installed.
Both have Wireguard configs, hence it should work fine when it comes down to speed (usual benchmarks go up to 850 Mbps usually).
Other alternatives are up-to date on this post of mine.
TLDR: I can let you know how it goes next month but there is no reason the Flint 2 wouldn’t work for you if you want to take the OpenWrt path.
You can also ask specific technical questions on their forum if you face any (and if they are not answered yet).