Is it currently possible to transit Bangkok suvarnabhumi airport on a separate ticket. In other words coming in to Bangkok on one flight and exiting to another country on another flight another ticket?

On the ticket must be the final destination, so the baggage go directly there. You can not check-inn outside

Even if I only have carry-on bag and I buy the ticket before arrive?

Suvarnabhumi departure hall on February 6th, 2022 at 1:17 p.m.  Most of the restaurants were closed.

On 3/2/2022 at 5:51 PM, PhineusFreek said:

Even if I only have carry-on bag and I buy the ticket before arrive?

No.  You will not be allowed to board the first aircraft as they are responsible for taking you to BKK and ensuring you meet the entry requirements.  Transitting passengers must be notified to the Thai authorities 24 hours in advance.

What is your origin? Destination?

 

Transiting SBIA/BKK on an int'l-int'l connection is best done on one single ticket, issued by the first carrier (this is the default), baggage can be checked through, and the connection must be 24 hours max. I'd aim for 4 - 12 hour layover. The second flight is ideally on the same carrier, but this is not a must-have. 

 

Have some transited BKK on separate tickets? Yes, but that requires you to believe reports.

 

Better to transit SIN, where possible?

 

I'd say just enter, then exit thailand - same day, but you'll have to factor in the day 1 package/cost (~ 5,000+) and any additional logistics.