Here's an example I found that was just posted a few hours ago on ASN. So it shows you how every immigration office is different. This office requires TWO witnesses, but they just need the ID cards and house books, not the people themselves.
So you get your first extension with 800,000 in the bank. The rules of the extension you just received say that you must keep your bank account at 800,000 and never let it drop below 400,000.
At the SAME TIME as you are getting your 1-year extension with money in the bank, you start doing monthly transfers of at least 65,000 baht from outside of Thailand. So now for this year you are doing BOTH. After 3 months of doing the monthly transfers, 3 months after you received your 1-year extension, you can withdraw 400,000 from your bank account if you want. Because the rules of the 1-year extension you received said your account can never drop below 400,000 after the first 3 months. Then you continue doing 65,000 baht transfers every month while keeping your account at 400,000 for the remaining 9 months. Then after you have the 12 months of bank transfers and your extension is due for the following year, you go to immigration and apply for the extension showing the 12 months of bank transfers, as the 3 months of 800,000 and 9 months of 400,000 to meet last year's rules. Then once they give you the new extension you no longer have to keep any money in the bank, but you need to continue doing the monthly transfers forever.
What you do with that 800,000 or 400,000 is up to you, but it's not required for immigration once you make the switch for year 2.
You being with him probably won't make a difference. For the first 2 trips, were they maxxed out trips? (60 days + 30 day extension?) The extensions are what really seem to get immigration interested in someone.
The exit ticket is good and important to have, but the other 2 proofs are also important to have (20,000 baht or equivalent in another major currency IN CASH, and proof of a hotel booking for a week or two at minimum).
Some offices require it and some don't. For the ones that do, normally they just want you to get a copy of the ID card and house book of a Thai person, that person doesn't actually have to go to immigration with you. I'm not sure if there's a form for it or not though.
You could do that, but if you ever need to go back to your bank and they want to look at your passport, they could close your account. Or if they decide to do a new wave of KYC in the future and freeze accounts until the owners come in for verification, you could lose it then as well.