Hi op, my experience is that 3-4 hours - day or night was good, plus we had period of quick top ups - that annoying thing as you say, of having an extra bit of milk an hour after a feed, or a couple of hours of cluster feeding in the evenings. I didn't mean it to sound so matter of fact!
A routine to your DS might just be what he's doing- I appreciate that makes it a pita for you!
Have you mapped out when he feeds over the course of a few days, it there more of a pattern to it than perhaps it first appears? This might help you 'accept' this is his routine for now. He might be changing this natural feeding pattern which could be throwing things off kilter.
Are there times when he's having a comfort top up rather than a real feed? Could you trying other things,not feeding, at these little feeds, if that is your hunch. Eg settle with a dummy(shoot me down in flames) if possible give DS to someone else to settle- they sniff milk and you at fifty paces. If they don't settle in a minute or two after that, then you feed...
As for combo feeding, I've not done it, I was just boob, but I would imagine he is getting different things and a feeling of fullness from different feeds. This might affect over the course of 24 hours how he feeds?
Lastly with the bottles, if you worked out how to get the best of both worlds, is the convenience of boob feeding in the day, instant milk, no sterilising etc, better? Then your evenings/ night could be formula - sterilised bottle, carton of pre prepared milk. Then you might be able to share the night time load a bit?
First feed of day, boob - i seem to remember being told this is the best quality milk of the day...