I just wanted to post an update on my issue. I've got rid of the "DMA Control Fail:01" error.
You were correct that the DMA controller chip needed replacing. I've done this and my M24 boots fully into DOS 3.3.
I didn't need the parallel code reader after all.

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Before starting the swap out, I checked the DMA pinout and confirmed my existing controller was getting the expected voltage. This is about the limit of electrical tests I can perform with my equipment.

In regards to the soldering, it is tricky if you're a novice like myself, but it is do-able.

Whilst I am a novice, I found the following useful (this is what worked for me).

- Using a temperature controlled soldering station with a suitable sized chisel tip (375 degrees C for desoldering and 350C for soldering). I picked up a cheap Yihua 936 unit and additional tips (don't use the provided conical tip).

- I chopped off the legs and de-soldered them separately. When doing this I put the iron on the remains of the leg and was able to lift most of the legs out without tweezers. Most of them adhered to the iron enough to lift them out, as long as the iron had some solder on it. But you’ll probably need tweezers for a few.

- To clear the through holes of solder I used freshly purchased (Chemtronics) solder-wick, any good brand should do. This was purchased through a known supplier as apparently there is lots of counterfeit solder wick (not all wick is created equal and old wick is less affective). I used just over a roll, so make sure you purchase enough, and ensure it’s not too wide. I tried a solder ‘sucker’ pump, but it didn’t work well for me (probably just personal preference)

- I used a quality turned pin IC socket, and was careful not to use to much solder when installing the socket.