It's a great compact car and offers a lot of bang for the buck. My wife wants one as her next (from her 540i).
But it's not an M-car. Yes, I have driven both and with respect to OP you can turn off the driver's aids so you don't let
them drive you. I want something more bespoke in engine build and design focus of M-div.
Also, and this is irrelevant to most but some do need a bit more interior size and at 6'' 3" I find the 1er cramped. I could totally go, though, for a stripper 1er (and I mean no luxo stuff) for track day fun. Just not a primary car. I went through the same logic over not choosing a 2010 Cayman S over an E92.
I enjoyed your review and I sincerely hope you have a blast in your modded 135i