Do I need a reentry permit and a COVID test to cross from Mae Sai to Tachilek for business?

Do you have a 1 year multiple entry non o visa ?

OR

a 1 year Extension based on a 90 day Non O visa?

you dont need the re entry to cross over:

BUT you need the re entry permit to re enter Thailand and keep ur Extension valid:

Accommodations are better in Mae Sai than Tachilek

Many recent reports are that vaccination and Covid proof is no longer checked when crossing into Myanmar at Tachilek. Officially, the requirement is still in place but seems to be enforced only by airlines flying into the country.

For a re-entry permit, you can get it at your local immigration or in Mae Sai. The Mae Sai office is 2 or 3km before the border, same side of the road. Expect a 2-3 hour wait if there's a few people inside the office.

Suggest you get your re-entry permit on day 1, stay the night in Mae Sai or nearby (Golden Triangle, 33km away has better accommodation) then come back the next day to cross the border. On the other hand, if you're intending on spending some time in Tachileik anyway you could always show up at Mae Sai immigration when they first open at 830am and you'll probably be out of there by 930am and in good time to cross the border having most of the day to spend in Tachileik for meetings/business etc.