Back in 2018 I entered Thailand on a tourist visa. I then applied for a non imm o visa thinking I had enough pages to get the non imm o then towards the end of the 90 days get my retirement extension before getting a new passport. The IO pointed out that I could not use the last few pages because, as the OP pointed out, they were for endorsements not visa stamps. I managed to get my non o then immediately got a new passport before applying for my first retirement extension.