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.