Electrical – Continuous Load Definition for Feeder Wire

That is correct. The 100A breaker in the main panel protects the wiring to the subpanel, and the subpanel itself. The "main" breaker in the subpanel is nothing but a shutoff switch (unless it is also a GFCI or something, as in the case of a hot tub panel).

The subpanel must be large enough to be protected by either its own main breaker (if it even has one) or the supply breaker in the main panel. In your case, a 60A subpanel would be wrong, but a 225A subpanel would be right.

The usual blunder people make is thinking the subpanel size must match the feed breaker, and thus they limit themselves to a panel with too few spaces.

So go for broke. 42 spaces is not too many. (Spaces are not circuits; a 40 “circuit" panel is often a 20-space). Good sign when spaces are a multiple of six, that indicates a panel of industrial quality (also sold in 3-phase).

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.