Will I face issues re-entering Thailand after a week in Cambodia with a prior 30-day stamp and extension?

No problem

Lisa Paul . . . the airline you intend to fly to Thailand one-way out of Cambodia, CAN ask you for an onward travel proof out of Thailand within the 30 days you will get stamped in. In this case you better show them a REAL one-way ticker with a budget airline, like Air Asia from Hat Yai to Kualy Lumpur. Maybe the cheap "onwardticket" is sufficinet, but we never guarantee this. You should know, Thai Immigration normally will not ask you for this proof and proofs of accomodation and 20,000 THB in cash (or equivalent in another currecny) - but the Airline in most cases will ask for onward travel proof, and if you can't prove it they can refuse you boarding the flight

No problem at all

Not if you can provide evidence of funds and maybe proof of acomadation don't worry enjoy your holiday

@Lisa *****

its up to immigration

Be fine just make sure you have a flight out of Thailand booked within the first 30 days as evidence.

Not unless you have a history of staying in Thailand for extended periods as a "tourist"

Not if that’s your only history of entries. If you have an extensive previous history then you may well have.