I have pink Thai ID it took one and half hours of phone calls by bank to receive 2nd Kasikorn bank account. It was much easier to get a bank account 2 years ago.
I only had lease and obtained COR able to obtain licence and purchase car before retirement visa. I used Laurie Chiang Mai visa shop never had any problems.
Paul what you write seems like a practical solution. Try it in real life. Pay 5000bht to agent problem solved if you have a 12 month lease or yellow book.
I tried many small branches on advice without success. One branch told me to marry Thai lady to open account. Ended up using Laurie, Chiang Mai visa shop with no problems and now have retirement visa.