Try doing through Mr Prakan instead. Maybe even a little cheaper although theyll only be open Monday-Friday Thai office hours so you'll have to wait until tomorrow
Which airline are you flying with? If one of the middle East airlines then you may be able to change your flight to their hub direct from Colombo rather than passing through Phuket again
It would have to be a ticket booked through. Sounds like you're booking flights independently so would be required to collect baggage and clear customs so a new Thailand pass and test and go would be required I believe
You flying from Heathrow? There's some places doing 2 hour PCR if you're prepared to pay I believe. Might be cheaper than changing flights despite the extra charge for fast turnaround
Book directly through hotel to ensure all boxes are checked. By all means use agoda and booking.com for research and, if you see cheaper prices there, try and get the hotel to price match direct. They'll still be quids in as not giving the booking services their cut