I'd be saying your issue might've been that you had no visa at all. Though I did the same, came here with no visa, and at the end of 90 days, I went to Vietnam for 4 days and returned without a visa. Luckily I didn't have any issues.
If you're only intending to stay upto 90 days, no need to pay for a visa. Enter on visa exempt, get 60 days. If you need more, extend for an additional 30 days
When i was at Jomtien for an extension, I had to renew my tm30. I thought maybe because I was ona visa exempt entry, so I clarified with the IO. Even if on a 1 year extension and leave Thailand, a new tm30 is required. That lesson cost 1600฿ 🤣
the main thing seems to be reasons to return to Thailand. The fact that she's had 2 already should make it a more straightforward process. I done letters from her family, letters from me, all financials. Only reason for going to UK is to spend time with you and your family.