note about the restrictions:
Transit passengers are allowed to enter and exit Singapore once during their layover period, as per guidelines stipulated by Immigration & Checkpoints Authority of Singapore. Therefore, passengers are not able to join more than 1 tour.
tour drops you off back at changi (no way around this), and afaik you can remain landside or go airside. tours can fill up quickly, so go register once you land
https://www.travelcodex.com/taking-f...hangi-airport/
The tour drops you off at the departures level at Changi airport, so from there, you are free to split or proceed to your onward flight.
I've stayed overnight at SIN twice before (flight at 6am), crashed at the various PP lounges (prefer the Plaza Premium one)
I recall the SilverKris lounge being busy around midnight (a lot of european flights).
you may want to reach there after 2am, though public transport stops around midnight. taxi/grab is cheap enough (under 30SGD )