Just my 2 cents, right now it is high season and the staff are probably very stressed. I would give them the benefit of the doubt, but yeah... I have also had my fair share of uneasy conversations at Immigration. I always feel tense and nervous interacting with them, and this mostly boils down to previous "interrogations" for staying "too long" in Thailand. Next trip I am aiming to arrive with a DTV to avoid all this.
As the OP wrote, if there only were some clear rules. Maybe the new ETA will bring some clarity. In the video below the guy on the left says that the alarm bells will start ringing after 3 VE:s per year, and then you get blocked by the system. After that you have to apply through the e-visa portal and obtain a real visa, not exempt. I don't know what he bases this on, but it seems he knows a bit more than usual.
You can have a look here: