It doesn’t void the warranty. In FPOOS (officially supported) it can even simply be enabled in the developer options.
Installing an alternative OS doesn’t void the warranty (FP made a conscious choice to ship the FP2 with an open bootloader), but if you contact support and they think you have a software issue they’ll probably ask you to go back to FP(O)OS to see if the issue persists.
But the FP2 port is officially supported by the LOS community and you’ll find help for most issues that might come up right here on the forum.
Unfortunately I believe another official G%§$e-approved upgrade is unlikely. But yeah, Lineage is a perfect alternative and will probably see many upgrades.
No, there will most likely not be any major upgrades. Well there might be an upgraded motherboard designed by a community member.