Nothing, you won't have any interaction with immigration. Now, of you want(need immigration services then your girlfriend should setup an online account and file the TM30 with the landlord credentials.
It's not really a minefield. There are rules and processes to follow. Can be a bit tedious, but very doable. All the minefield you hear are about people trying to circumvent the process, bribe officials through agents and not understanding what's required.
While your in pattaya, go to an immigration office and ask for handouts on requirements for visas and extensions of stay based on your age or martial status. There's a lot of noise in here and lots of repeat questions. If you read you will learn whose answers to rely on....
Be wary of "offers" that circumvent the rules. You're only exposing yourself to greater risk down the road.
many over 70 (typical expat on a non-o) will find health insurance very expensive with many exclusions. If over 70, expect to pay about ฿15k/month and may find that many issues will be excluded as preexisting conditions. Insurance here is archaic.
The point of the 800k is that it is to be accessible and liquid for medical (and other) emergencies. I don't believe a fixed account would qualify since the funds aren't available to you.