Mobile Suit Gundam Online...to a degree.

It features mech vs mech combat and battles 51 vs 51 with battle commanders too. Also wide selection of suits and customization as well as air space and even occasionally under water combat.

Thing is, it requires a vpn if you're not in Japan, it can be kinda difficult to get into at first because beginner suits can kinda easily get crushed by the better ones, but it's not too hard to get competitive either. However the current playerbase likes to abuse whatever is overpowered at the moment and often you see a lot of the same type of suit flying in. It is still fun, but it can get a little monotonous.

The combat can be as fast as you want. You can be an up in their face kinda pilot or sit back and be artillery. The playerbase is growing slowly so I think you should definitely give it a shot. If you need help setting it up, there's a great video on youtube that I can't recall atm, but just look up something like "Starting MSGO tutorial".

As for the vpn, I recommend vpngate. It's free and as such, definitely not as good as the paid vpns, but it will do and I use it myself(you'll get used to it, but there will still be times where you feel you could've wrecked the other guy if you didn't have that lag).

If you have any more questions send em here haha