Any question answered...

This sounds like when someone was planning on making their own garage doors. Good picture framers are not very common,like many skills you could probably do a reasonable job yourself. However I assume there are tricks to the trade which will result in a better job and being done faster.

3672x2721km

That is pretty big.

Yes. Give it a go. I've been making frames for a while. Careful measurement of length and angles and you'll be ok.

You might want to router a channel into the back for whatever backing you want to use.

I have a decent saw and a mitre - I'm not an idiot. Is it reasonable to assume I can make a frame myself?

I think it's the design X materials which will dictate whether you can do it yourself.

If you're happy with a chunky wood frame, then you'll be fine. I think the problem comes when you want something slimmer and require stronger materials which don't flex or warp over the larger distance.

Corner clamps / frame clamps are available at a variety of outlets for a variety of prices - Lidl usually have them once a year at least. They will make your job much easier.

However, if you want picture framing glass you will have to buy that from a picture framer, and if you want anti reflective glass it will be pricey, so why not give them the job of framing it while you are there - plus you don't have to worry about getting a large, thin, expensive piece of glass home on it's own.

Alternatively, A0 is 841 x 1189 mm, which is only 200 mm longer than your map, so you could just buy an off the peg clip frame and throw it in, with a large bottom border.