I think the OP is referring to 500,000 baht in the bank. If you are having to borrow 15,000 bht you are probably a mule. And if you don't have the 500k, you're one of the reasons why some Thais want this visa gone.
All relationships are temporarily. Eventually one partner ends it by dying. He's married, and had been married for a substantive period. Sounds to me they had dotted the i's and crossed the t's but were unlucky in that the embassy official had met their quota and got to keep their own job. They too will have measures about how well they are doing their jobs.
I've been told that the UK looks upon favourably Singapore, Hong Kong and, surprisingly, Vietnam travel. My wife has an essentially criminal record for overstay in Malaysia, but, least last year, for the UK, that wasn't any issue. We ignored the UK lawyers who told us to lie about the past.
In the unlikely event of a new member ever reading this thread, a suggestion would to start a question by "I have searched the archived postings but cannot find the answer I need. (Blame Facebook, etc). So.....". You don't have to actually search, just say you have.
Agents can help make sure applications are completed correctly, and put in the best possible light. I recommend Visa for You for assisting my wife get a UK visa in spite of a 2019 Malaysian immigration conviction. There are a few unrelated companies with similar names, so see the screenshot of their FB account to match them. All the others, including a UK based outfit, suggested lying to the UK Home Office about past travel history. We just fronted up and see where the sticks fell. The agents will suggest she has 100k in her bank account for a few months before application. I don't know if they are any better than others, but worked for me. Some others gave a Thai price versus Farang price. I would suggest letting your girlfriend find one herself. If she is from Isaan, they seem to like dealing with other Isaan people.if she gets turned down, and you've picked the agent, that might sour things, even though the agent can't actually affect the outcome. If she picks the agent herself, what you can do is to check if they are legit. If it goes wrong, no comeback to you. You probsbly find you will have to marry her. Most won't have a money back guarantee but will recycle the fee into a fresh application.
Thailand will only know about your travel to Thailand. The passport itself only contains information about your identity. Thailand will have no idea about other travel history. That's not the case for other countries, where there is a lot more sharing of passport data. Thailand I think is part of an ASEAN agreement to share data about ASEAN passport holders. The Migration 5 or 5 Eyes do more intrusive data sharing.