Electrical – Separate panel for house in garage – how much capacity do I reasonably have left

To answer your biggest question: I'm pretty sure it's not code-legal to tap into the feed from your meter panel with split-bolt connections. It's possible that your breaker panel has some lugs near the bottom of the load side bus bars to connect a feed to a subpanel, but most don't.

So a possible solution is what Doxy suggests: a new 100A panel that feeds your existing panel as a sub-panel. Before you consider that, I'd check to see if your panel type admits half-size tandem breakers. These are breakers that fit two half-size breakers into the space for one normal breaker, and this would free enough space for a 40A breaker to your EV charging circuit in the garage.

Another possible difficulty, though, is that your 100A service is awfully marginal for the addition for a 40A EV charging circuit. Although you didn't give all the relevant details, I assume your 1000 sq ft house (does this include the basement, if there is one?) has an electric dryer. In my jurisdiction, a load calculation shows you would require a 107A service. You may be able to get away with a 100A service, but it will depend on your inspector/permitting authority.

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.