Welcome to the forum. It’s pretty quiet around here, but hopefully as more people join it’ll liven up. I’ve also been a contributor and reader of many forums and consider them excellent resources, so the more the merrier!
Haha I've been a BMW owner for years. I recently sold my 2007 3 series that I had for almost 13 years (bought lightly used) and it was solid. The M4 remains in the stable and is just too much fun to let go. It has the carbon fiber bucket seats as well. I ordered to my spec and it was produced and delivered about 3 months later. The cutout in the trunk where a spare could "almost" fit is actually where the battery is. It would have to sit on top with the luggage and isn't worth the sacrifice in cargo space. It's easier just to have the CX-50 now for those trips, especially with the unpredictability of the weather here in the Carolinas.Congrats and welcome!
No wonder you are here after that pesky BMW left you stranded. I am surprised you still own it. haha Are there spare tires available or spares from an older BMW that may fit? If so, maybe worth it to buy one and throw in the trunk for longer trips where you might not want to take the Mazda? I own an MX-5 and this is what a lot of fellow MX-5 owners do. The downside is lost trunk space but at least you have a back seat to make up for it.