The flight aspect of independent travel in Thailand as you may have known it in the past is completely gone.

With "unconnected tickets" your itinerary will be seen to end in Thailand, regardless of your intention to not leave the airport. (It doesn't matter whether you intend to fly on to another location in Thailand, vs. to another country.) You won't even be able to board your flight to BKK unless you have arranged one of a couple different options for covid protocol -- all of which involve at least one night (and possibly 7 or more, depending on your vaccination status, where you've been in the previous weeks, your nationality, which quarantine scheme you use, etc.) in a hotel you have pre-arranged, and at least one PCR test.

I'm actually not sure whether international transit is allowed in Bangkok given the current hysteria. Someone else can comment -- but to even consider that option you would almost certainly need each direction on the entire journey to be on a single booking, e.g. LHR-BKK-SIN and SIN-BKK-LHR could possibly be separate bookings, but you can't have separate bookings LHR-BKK and BKK-SIN.