demue

You mean SQ Eco I assume as SA is the airline code for South African Airways. Anyways, whether SQ Eco or Scoot Plus (Biz) is enterly your call, both are perfectly acceptable for a 2.5hrs flight.

AFAIK, if you take Scoot, a transfer (within T1) would not be possible as you have to collect your luggage after your JAL flight (no interline). That means you have to go through immigration in T1, collect luggage, go to Scoot check in, go through immigration again. As bad as it sounds, at Changi Airport this is entirely possible without rushing within the time you have (can probably done in under an hour), assuming your JL flight arrives on time.

There are no airline "arrival" lounges in the "transit area" in Changi to my understanding so your JL Biz arriving flight BP means nothing. They are departure lounges, so lounge access is based on your departing flight. Unless you are *A Gold (if you fly SQ) a SQ Eco ticket doesn't come with lounge access. ScootBiz also doesn't include lounge access IIRC. So you can only use lounges via lounge membership like Priority Pass, Dragon pass, Loungekey etc. For that any departing BP will do.

If you go SQ route and your luggage is checked through by JL all the way to DPS (Bali) then for a change between T1 & T2 you do not need to pass through immigration. You just take the train between terminals and stay "airside", there will be a security check at the gate, that is the way it is at Changi for all flights (no centralised security). As always, if your JL flight is on time, the switch between T1-T2 would take 15-20mins max in my book even if you don't know Changi Airport well. Whatever the person on TripAdvisor stated does not make sense to me, a lot of rubbish advise is floating around there, but I digress.

How much buffer you would like to have between flights to be comfortable is an entirely different story, as you are on two separate tickets. Some people may be okay with 1hr only while others will prefer 3-4hrs or so. The risk here (at Changi) is not an inefficient, cumbersome airport like in so many parts of the world, the risk is primarily late incoming flight into SIN. Only you can answer that question for yourself.
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If you misconnect on two independent itineraries, depending on the fare you bought, SQ or TR (Scoot) could force you to buy a new ticket to get onto the next flight to DPS, that is the risk. Stuff like same day standby or so like what some US carriers offer is not a thing with SQ/TR unless you bought flexible fares. YMMV.