My advice for what it is worth based on personal experience. I used an agent to obtain extension based on being over 50 twice mainly due to covid trapping me in Thailand unexpectedly and I had not got access to the required funds at the time. My local immigration office did not like it. They made my life difficult over a number of things like residence certificates and transferring stamp into new passport. They realy gave me the run around. I was also pulled aside at bkk on entry and told in no uncertain terms that I should get my next extension myself. I opened a second bank account into which I put
*****
0 baht 3 months prior to applying myself at local immigration for my new extension. I was given this with no questions asked about funds for my previous extension for which I had not had the required funds in my account for the required time. The IO did not ask to see any proof in my first bank account. At that time I started monthly payments into Bangkok Bank of
*****
baht and this February obtained my new extension based on those. I now go forward with monthly payments which can be spent or sent back to UK to send again next month. There is a cost involved in that but minimal and it releases the deposit of
*****
0 baht which I will invest at a much better rate. I have had no problems with Wise transfers showing as international but the IO required my UK statements to verify origin of transfers.
The icing on the cake is that now the IO at my local office could not be more helpful and has offered helpful advice about several issues I have had.
So, in all, things have been much better for me since getting my extension myself and I save a lot of money as well.
Make of that what you will and good luck.