Can I obtain an extension on my tourist visa in Thailand after being denied initially by the immigration officer?

First of all I would like to thank all moderators of this group, especially Tod Daniels, for their incredible work providing such professional and reliable advice for free to the community. You guys are simply amazing 🙏🙏🙏

I am looking for some confirmation on my side.

I am a French citizen, 43y old, not married to a Thai wife nor having a Thai kid.

I am staying in Samui, where I bought a house 1y ago, with my Hong Kong wife and our daughter.

I stayed almost 1y with an ED visa (with a border bounce at Malaysia border (Satun) at the beginning to save some time).

I am currently otw to get a working permit for my (future) business and I just got a Tourist visa in Phnom Penh to stay 3 more months in the meantime.

My tourist visa is valid until 13th October. I got it stamped yesterday at Bangkok airport immigration for 60 days (until 17 Sept).

I understand I can claim an extension at the Samui Immigration office to stay in Thailand until the last day of my visa (13th Oct) which is equivalent to 26 days extension (due to the few days to "process" the visa at Phnom Penh, since they use the application date at the starting day of the visa...)

My understanding of the situation is getting challenged. Can you please let me know if I miss anything?

Thank you so much again 🙏