it’s pretty quick it takes approximately 4 to 6 weeks. I use an agent it’s a lot faster. You get the 30 day tourist visa upon arriving there’s rumors are supposed to go to 60 days but it’s still 30 days as of now and they may have to do one extension during the process but overall it’s pretty quick. 
yes I’d rather donate in Pattaya as it’s more convenient unless I happen to be in Bangkok at the time I have no problem donating up there on my next trip as well 
it depends what city you are in and what immigration center you go to I’ve seen over two hour lines before plus i don’t have to do the 90 day reporting like you do my agent does it included.
Just be careful because if you do a language course for a lengthy period of time and you leave the country, Thai immigration will start quizzing you in Thai as you should know how to speak it after a few years that is how they busted all the Chinese that were there for like nine years on education bases, and did not know how to speak anything while back. 
not really that expensive it’s the non-O retirement visa and not much more than doing it yourself. I prefer agents that way you don’t have to stand in line and deal with all the BS. They handle everything for you.