If I enter visa exempt later this month, extend, then go to Langkawi for a week 2 weeks into the extension (so after about 45 days), then get the ferry to Phuket and enter visa exempt again, am I likely to have a problem? I had intended to get an SETV but it turns out I can't get one locally (because I'm not resident).  I last entered Thailand in December 2022 and stayed until 20 Feb this year (visa exempt).

Shouldn’t be a problem. I have done that route a few times and Koh Lipe immigration is quite straightforward