[X Style (Pure) Accessories] CthulhuDreams:
So I did the repair, took me far longer than I would have liked. Here's my experience in case the information helps anyone else:

To remove the back cover I used the Motorola SIM tool that came with the phone, and while I was able to pry the back off using the tool and a heat gun I borrowed from a friend the tool is now bent (cheap to replace) but it also bent the phone frame every so slightly where the opening was. It's not noticeable unless you look closely, though I can definitely feel it when I run my finger over the spot.

The most time consuming part for me was trying to remove all the left over adhesive from the phone and back cover. It took me far longer than I'd like to admit before I was happy with my efforts. On the phone itself I used a plastic opening tool to scrape the adhesive being very careful not to hit the ribbon cable between the buttons and camera. On the back cover I used the same tool as well as some Goo Gone, which was much easier than scarping alone with the phone. I considered dipping a cotton swab in some of the solution and then wiping the phone as well but decided I'd rather not risk a liquid near the phone itself and stuck with the scraping approach.

I used a Wiha T3 Torx screwdriver and it worked perfectly, no problem getting the screws out. They were in fairly tight but the driver handle it pretty easy. Took a matter of minutes to get all 20 black screws out.

Separating the two halves took a little effort as one of the two retaining clips just didn't seem to want unfasten, and the other one didn't really fasten properly when I put the phone back together.

I took a picture of the NFC contacts on the current battery so I could place it onto the same spot when I swapped in the new battery. Getting the old battery out took a bit of doing. I did try using the pull tabs, but on one side only about half of it came out so I had to do some prying to separate the battery from the back plate. I was also careful not to tug too hard at the point where the battery was separating from the back plate near the side button's ribbon cable anchor point. There's a bit of adhesive holding that junction in place and if your're not careful you can either pull the connection out and need to reseat it or even worse actually tear the cable itself. Thankfully I manged to pull the battery out without disconnecting or damaging the cable.

The new battery I got was from Mobile Defenders, which from what I read was an authorized Motorola service provider at one point and thus stocks genuine parts. I can't link it, but if you search for FX30 it should come up. Shipping was a bit pricey at $10, as they shipped it FedEx ground properly marked as containing a Lithium battery. This battery was curved, just like my old one and had the same markings (nothing like 'Engineering Sample'). I secured it back into the frame with some 3mm double sided tape I got off Amazon and then stuck the NFC contact back onto the same spot on the new battery.

I then reassembled the two halves and reinstalled the screws. I did some quick checks to make sure the phone charged, powered on, and still had NFC capabilities.

Satisfied that everything was working I then used a die cut piece of doubled sided adhesive I got off Amazon to reattach my original back cover to the back plate.

The new battery seems much improved, though I'll have put it through a few cycles to confirm. AccuBattery reports the charging capacity as over 3000 mAh. Though the actual battery health number hasn't gone up to that yet, it has been slowly increasing since installing the new battery though.

This was the first time I'd ever done something like this, it wasn't too bad, besides all the issues caused by the adhesives. It also made me realized how much I miss easily replaceable cell phone batteries like in my earliest Android phones.