90 minutes isnt a problem. Your bags will be tagged through, and you will just clear a security check at BKK before being back airside. Even if you have the longest possible walks, you can make the connection in ~40 minutes with time to spare. 90, and you'll have enough time to visit a lounge (or two)
90 min is more than adequate at BKK - even when you are on 2 separate tickets. Dont worry!
I understand that you prefer to skip BKK-SIN on your onward connection/separate ticket and fly only SIN-LAX (and presumably DPS-SIN rather than DPS-BKK).
This will not work. If you no-show for BKK-SIN, all remaining flights will be cancelled (incl. SIN-LAX). There is no other way than to fly DPS-BKK-SIN-LAX, sorry. It is not possible to cancel only the first leg of your onward ticket.
Will the travel be before 10th August? One of the runways at BKK will be closed between now and then, and this could cause delays, and may be worth factoring into your decision.
Also: if you do have the flights on separate tickets and will have checked luggage - a question worth asking - will TG definitely interline bags to SQ? I know they are both in star alliance, but I have encountered some carriers who won't interline to other star carriers if on different tickets. I have feeling TG do, I am sure others can advise with more certainty if they do or don't. (If they don't interline and you will have checked luggage - 90 mins is definitely not long enough at BKK).