I'm not sure what you mean by "activate it" but the 5 year visa validity starts on the day you were approved.

Then each time you enter Thailand you receive a 180 day entry stamp.

Come and go as often as you like during the 5 years. Each time you receive 180 day entry stamp.

Edit: of course any and every entry is up to the immigration officer and depending on your specific circumstances, the port of entry, which type of DTV, the mood of the IO, and many other factors, entrance is never guaranteed for ANY visa including the DTV.

I have entered and exited a couple times already on the DTV (remote work nomad). Never had any questions from immigration. I went to Taiwan, Spain and Laos.

I definitely prefer airport immigration over the land border at Vientiane because much easier than the walking, crowded bus between Lao and Thai border, etc

The other country didn't ask any questions related to the stay in Thailand or DTV. Irrelevant to them

It sounds like you possibly have a misunderstanding of the DTV...

The 5 year clock has already started on your DTV from the day it was issued. When you enter Thailand for the first time is irrelevant for this.

You can enter Thailand as many times as you want during the 5 year validity.

Each time you enter, you receive a 180 day entry stamp. You can enter as many times as you want.

And finally, you can not have multiple visas at once. Now that you have the DTV, then this is what is used when you enter Thailand

Customers are Thai. Not allowed as a remote worker. Need a work permit to do this

Come and go as often as you want during the 5 years and get a 180 day stamp each time.

I did it in Jomtien. Need residence certificate and copies of relevant pages in passport. Also plan on having to go back a few times...

I would suggest going to one of the inspection stations to do it rather than the main office. No long lines, they handle everything, can get the insurance there also. For me it was 30 minutes closer drive ad well.

No need to change anything. The only time this could become a cause for question is if you want to do an in-country extension rather than a border run.

What does it say in your passport for place of issue? This is what you're supposed to input.

Extension or do a quick border run at some point during the 180 days. I prefer border runs

Officially not married so not a spouse. So both of you would need a separate application and qualify on their own.

I don't quite understand your comment on the bank account either... The bank account doesn't need to be in Thailand. Your partner couldn't open a bank in their home country or other country where you are?

Either way, to qualify each applicant has to be able to show a bank account in their name with 500k baht. Supposedly some say there is a way for a "sponsor letter" but haven't seen any reports of it actually being done