Look at LAX/CGK, overnight at transit hotel in airport, then morning flight CGK/UPG/MDC the next morning on Garuda. I've done this the past few years & it works fine. Garuda has an extra 15kg scuba/sporting equipment allowance, but you have to ask for it & maybe talk to more than one agent Jakarta Airport has been upgraded over the past couple of years. It has an international terminal & a domestic one, but the good thing is all Garuda flights depart from international (which is just downstairs from transit hotel). I've found this to be very convenient.
I've been flying EVA Airlines LAX/TPE/CGK. Good connecting times in Taipei.
P.S. I read on my Nov trip to Indonesia that Singapore was discontinuing their non-stops LAX/SIN, so that may be an issue.