I’m planning to fly BKK/Phuket RT near end of April. I don’t have proof of vax on an app, but do have physical proof of vax plus the document acknowledging same when I entered in September. Is there a booster requirement?
Is there anything else Covid related I need to do to fly domestically?  
Thanks in advance 

We flew from Chiang Mai via Bangkok to Phuket last Sunday, and absolutely nothing was required or checked. Just boarding pass compared with Passport or ID.

In my multiple flights to Phuket (but only to Phuket) I've always been asked for proof of vaccination at the gate (since I do online check in). The agent will accept about anything as proof. At different times I've used my (US) physical card, my iPhone VaxYes digital card or my US State vaccination digital card. 

 

There are thousands of different vaccination documents. Airline gate agents aren't knowledgeable about what they are looking at and will accept almost anything. 

Flew last week Vietjet to and from CNX and BKK and nothing was required.  Flew Bangkok Air to and from Trat and I was requested to show my vaccination card (CDC card from US) when checking in for the flight to Trat.  Answer to OP's question seems to be "maybe" so wise to take your proof of vaccination along. 

TO Bangkok, normally nothing required.

 

FROM Bangkok to most internal destinations, proof of vaccination or negative test required.

 

Check the airline websites, the info is all there.

If you have proof of vaccination from entering in September you are OK.

No booster requirement.

I flew to Phuket in December with AirAsia and nothing has been asked/checked.

(but occasionally there a post saying that vaccination proof had to be shown).

As already written: nothing required for flight to Bangkok.

Bangkok to Khon Khen two days ago proof of vax required, mask on all the way, no food or drink on the plane.