The discussion in the comments revealed that the "planned addition" as described is perfectly fine but that the only detail that really needed additional confirmation was the conduit fill calculations, specifically regarding derating.
Since I'm using a T-style conduit body to branch a raceway for two fixtures there is not room to have a code-compliant splice between the common junction box and the fixtures themselves. This then means that separate neutrals need to be pulled and that there will therefore be more than 3 conductors in the shared part of the raceway. I had initially read that derating didn't come into effect for small wires (#14 and #12) until a much higher number of conductors are involved, but it was a good exercise to truly confirm this.
The NEC requires derating for 3-or-more conductors in one raceway. My initial impression for #14 THHN was that it's rated to only 15 amps, which would then mean that any derating requirements could be an issue for this 15 amp branch circuit. Looking deeper tough it turns out that 15 amps is only the required overcurrnet protection for circuits with #14 wire (from NEC from 240.4(D)(4)) and that #14 THHN is actually rated slightly above 15 amps at low temperatures. In my case I needed to derate 80% (for 4-6 conductors), and for #14 THHN at low temps this worked out to 80% of 25 amps = 20 amps, which still appears quite fine for the existing 15 amp branch circuit that I'm working with. This handy online tool, and of course the appropriate tables from the NEC (from 310.15(B)(16)), confirmed this.
So it looks like this planned addition is fine as stated assuming a wire gauge <= #14.
"Future capacity" is job one when installing a panel. Think big. Make sure you never run out of spaces. Small "main breaker" panels tend to have very few spaces, so I recommend aiming for spaces and disregarding main breaker or panel capacity. It doesn't matter anyway; read on.
6/3 NM-B is the legal minimum cable for that run. You are a long way from worrying about voltage drop for distance reasons (I don't even bother checking til 110'). You don't need to worry about "wet locations" so NM-B will suffice.
Cables in a ceiling space are usually not a problem, but there is some technical stuff about plenums (air handling spaces) that I'm not up to speed on because I just avoid them.
The subpanel does not need any disconnect switch since it's inside the same building. You might as well use a main-lug panel. Just FYI, if you did need a disconnect, and you used a main breaker, size doesn't matter. What matters is the feed breaker. There's no practical way to coordinate trips so the nearer breaker trips first, if that's what you're thinking. A 30A feed supplying a 225A sub is fine.
Copper #6 or aluminum #4 is officially rated for 55 amps. 50A is fine. However *you are allowed to "round up" to the next breaker size, so 60A is a free upgrade!
Going up, #4 copper is good for 70A only. #2 aluminum is good for 80A (75, round up) and #1 aluminum is good for 100A. (at 100A, you get a free "bump" because you're now allowed to use the 75C thermal column in the big temperature/ampacity chart. Aluminum is much cheaper and perfectly safe for large feeders like this; just use the goop and torque to spec.
Look at your excellent diagram. Note how you have the two subpanel breakers abreast. In normal panels, both of these are clipping onto the same panel bus bars, which for some reason are called "stabs". Now you have to start worrying about stab limits which are stated in your panel instructions. I recommend dodging this issue, by staggering these so your subpanel feeds are sharing their stabs with ordinary 15/20A branch circuit breakers.
Best Answer
Don't make your life harder than it needs to be!
First off, shucking NM of its jacket is no good, because the wires inside NM cable aren't marked properly for individual use. Second, pulling NM through outdoor conduit is unacceptable because NM is no good in wet areas due to the paper separator layer between the jacket and the wires (which wicks moisture up the cable, leading to hard-to-troubleshoot failures later), and all outdoor conduit is considered a wet location by NEC 300.9:
Third, there's absolutely no reason to make your life this hard: pulling 4 6AWG THHN wires through your conduit (hot, hot, neutral, ground) will be far easier than trying to jam a cable through there. So, I would simply get an appropriate amount of 6AWG THHN (black, white, green is fine) and use that instead.