Hi Federal Farmer
There are a few other small things worth pointing out.
R4 doesn't need to be so large, you probably don't even need it, it is only for dissipating the energy from the gate in each switch. With that large package fet, I don't think you need it.
It is also important to match Q1, Q2 and Q3 so that the bjts change charge up/down the gate as fast as possible.
I am not an expert on this, but I have been looking into it for a while.
After choosing your topology, I think the general plan is select your fet next. You want a safe voltage margin, (100v) is probably excessive, low RDS on and a small gate charge. I ignore the power or current rating as I am trying to get the absolute minimum power loss anyway, so it should be running fairly cool anyway.
Obviously N Channel fets can give better rds(on) values here, but you would have to flip the topology about.
You are using what is often called a Hysteretic buck converter, the hysteris should be in the comparator, it's primary advantage is that it minimises the number of switches for the amount of ripple. Therefore can be very efficient.
It's chief disadvantages are you can't easily control the minimum switching rate, so you may get flicker, especially if you add a dimmer.
The BJTs might need a bit more oomph (Imax) to drive a large fet, (large gate charge), maybe BC639, BC640 they were suggested to me.
Another thing to watch is you don't want to drive either of the BJTs into heavy saturation, so you should limit the base current so it just goes into saturation, and then it can come out of saturation faster.
You can model these things in LTSpice and look at the waveforms on a cro, what you want are nice square edges, as much as possible.
Two other lossy places are the current sense resistors, can they somehow be smaller?
and the inductor.
You might be able to use a smaller inductor depending on the allowable ripple, the smaller inductor will have lower series resistance generally.
The bigger the footprint too will give lower series resistance.
You can keep reducing until you are hitting the absolute maximum led current. While in development I would put fuses on the leds as they are easy to blow.
Probably the most efficient thing you can do is somehow use a lot of Leds in series, this would require a higher voltage circuit and a redesign, with a lot of leds in series you can easily get 90%.
Here is a link to my led project
https://www.eevblog.com/forum/projects-designs-and-technical-stuff/domestic-led-lighting-project/Good luck.
ps What sort of leds?