Also worth noting (separately) -

If you do happen to have two separate bookings on Oneworld carriers, Qatar can and will check you all the way through. The first agent (business class and, I assume, better trained) said it couldn't be done, ditto the second. The third said the same but then made a call when I insisted politely that it was something do-able. Lo and behold, I was right. She processed everything with a bit of help from my limited experience doing it before.

This meant not having to go through Customs and Immigration, collect bags, and recheck in Singapore. Instead we could go straight to the lounge for a shower, meal, and rest. Also, we had technical difficulties on our flight to Singapore & having the itineraries connected meant we'd more easily have been rebooked had we been really delayed. Thankfully the delay was only an hour and we had a 4 hour layover in Singapore.