I will do my best here.
I take it the reason you are asking about using NM wire is to save money?
Purchasing NM is much cheaper. You can still run NM inside the conduct if there is
room left. So if your conduit is 3/4 inch. you maybe able to pull 14/2 wire through with Lube.
Your subpanel would need a separate ground bus since the conduct is considered the (Ground)
By installing a Ground bus and using the NM 14/2 or 12/2 you must connect the Neutral and Ground separate as for the panel is not bonded. I believe the term is floating ground and neutral. Your Subpanel is connected to your Building since it is not Bonded. The question i have is why is your panel has only 1 bus bar? There should be 2 one for the Neutral and the other for the Ground. Your subpanel should have 1 Aluminum service line that contains 2 hots leads and 1 neutral and 1 ground. The other question i have is the service line coming from the Main Panel inside metal conduct as well?
The gang boxes are grounded by the Conduct, but I would recommend you put a Pig Tail in the box.
That is an old "rule of six" panel, which while grandfathered, is illegal under its grandfathering becuse it has 7 main breakers. Going to five is a good plan.
It is a classic "CH" panel which is a very good industrial grade panel, except that the 3/4" breaker width make non-ordinary breakers very expensive (a trait it shares with Square D QO). That makes it perfect for what you plan.
On your subpanel which would be near this panel, I would get a panel with a main breaker, with an eye toward (at some point in the future) cutting it over to be the main panel. In a subpanel, the "main breaker" is nothing more than an on/off switch, it is OK for it to be larger than the feeding breaker.
I would also get a rather large panel, at the very least 42 space and even 60 or 84 if practicable: because panel spaces are dirt cheap and often even come with free breakers, whereas running out of space is painfully expensive.
I would aim for an industrial grade panel of good repute (one available in 3-phase variants, not Homeline, BR, or second tier brands) and avoid the expensive 3/4" breakers (not CH or QO).
Over time, as you find it convenient, i'd migrate all your 1-pole and smaller 2-pole circuits over to the new panel.
For your garage panel anything would do, but I'd go for the same type as your indoor panel, so you can use some of those bonus breakers. Again it's false economy to scrimp on spaces, I'd go 20-30 at least.
Also, since garage spaces need to be on GFCI, consider getting a subpanel which has a "main breaker" which is GFCI, that way all the breakers in that panel would be protected (at the cost of potential nuisance trips, a big deal if you keep a freezer in the garage).
Ed Beal raises some very good concerns about overall capacity. One problem with these "rule of six" panels is there is literally no main breaker to stop you from drawing more than 150A. So it pays to be conservative.
It's a difficult situation because you have two big loads that operate sporadically - the EV charger and the range. And the A/C as a wildcard.
One thing I might suggest, is feed the garage subpanel from the new primary subpanel. And then move everything but the range over to the new subpanel. At that point the only things still in the CH panel would be a 60A range breaker and a 100A subpanel breaker. Even at max, those two could not overload the 150A service (by enough to matter). This would force your entire house (from A/C to EV charger) to share 100A, but would remove the possibility of an overload. This would also save you the $85 you'll spend on a second 100A CH breaker.
Best Answer
Go for it -- just don't starve new the panel for slots!
You have plenty of headroom on your existing service to add the new feeder (the 220.82 calculation for your existing house comes out to a mere 45A, assuming your kitchen receptacles are on 2 circuits and your A/C is reasonably modern/efficient as I used a representative 13 SEER for calculation purposes), and a 60A feeder is more than enough for your guest suite, as NEC 220.82 pegs it at a mere 29A (given that it doesn't have its own laundry facilities -- even if it did, it'd be only 35A).
The wire's fine, too. Even though 6/3 NM is only rated for 55A, you can't get 55A breakers, so the 60A breaker is fine as per 240.4(B), and the Article 230 provisions for service sizing don't apply to feeders.
One suggestion I will make here, though, is not to starve the new subpanel for slots -- something like a Siemens P3040L1125CU would be ideal here, even if it seems like massive overkill at first. That way, there's ample space to arrange your load across circuits how you see fit. Pulling the tie bar on the new subpanel here is precisely the correct move, as well -- neutral and ground never meet again, after all.