Not all countries offer that. Canadian passport used to be 24 to 48 pages. They changed it to a fixed 36 pages for everyone with no choice. I hate it, it's the bane of my existence lol.
I don't know for Thailand but as I remember I applied for evisa to Vietnam from China back then as a Canadian and there was no problem. Then I applied for evisa from Vietnam to Cambodia and there was no problem.
Isn't the whole purpose of evisa to avoid visiting embassy etc? Since when do you need to do it from your home base? Unless it's specific to Thailand.
I might just go next week for immigration because I heard they give you 20 days from the day you present at immigration. I still have until the 9th so going on Monday makes more sense