Expect to have that issue get worse there have been complaints that some epic mechs feel stronger than a lot of legendaries they are in a sense punching above their weight and plarium has acknowledged that as a "problem" that they're working on. Last time they "fixed" the lines between categories being blurry being a "problem" they changed the game so rare mechs got one shot by legendary mechs damage abilities at the same rank but they can still get matched together to this day. So expect a bunch of the best epic mechs to get hard NERFed or worse yet a legendary buff across the board so all your lower grade mechs get one shot soon.
And the matchmaking algorithm doesn't match you based on your equals. You are expecting it is meant to match you with your equals in some way that's why it looks like it's based on inventory power to you Instead what's happening is you were just using legendary mechs and doing alright so based on your performance that's where the matchmaker thinks you should be and it needs time to adjust. If you wanna win as much as possible while having fun that's just not an option right now. Maybe it would be if not for the no skill spender buff mech and weapon grade update. Seriously the worst update in the history of mech arena needs to be reversed not done again but it's going to be done again.
Since you're done spending, expect to start losing a lot more. I don't think you can exactly "become f2p" on an account you already spent money on, but that's where your status in the matchmaking algorithm will go. You can kiss fair competition with even your best builds goodbye. You're to be demoted.