Running #4 3 wire copper SER with ground
My understanding is that even though #4 copper is rated at 90 degrees, the breaker knocks it down to 75 degrees (confirmed on my breaker), which should be 85 amp service for SER
That's correct. However #4 copper doesn't have any rating at all. Cable types are actually insulation types. The insulation decides the thermal rating. Across the top of Table 310.15(B)(16), you see the various cable types mentioned in the columns. If the cable type isn't listed there, its thermal limit is specified in its section in NEC Chapter 3.
It was taken as canonical that SE cable was good for 75C for 50 years, until somebody at a cable manufacturer proposed a NEC change to limit it to 60C (because NM cable is, and no more logic than that). It came very late in the NEC adoption cycle - nobody argued because nobody noticed, so it was rubber-stamped. Once published, there was a firestorm of debate that resulted in it changes every edition, with the 75C crowd largely winning.
80 amp breaker on main panel (fed by 200 amp service)
Your wire is good for 85A and you may provision up to 85A of loads. 85A breakers are not made, and this is reconciled by letting you "round up" to the next available breaker. 90A breaker.
You're always allowed to use a smaller breaker, just watch your ground size. If the effect is to use larger wire than the minimum for that breaker, you must also enlarge your ground wire in proportion. There are cases where this can bite you.
Garage is detached (although can be reached through crawlspace)
Either it's attached or detached. "Having a breezeway connecting them" is the low bar for "attached". A breezeway is a passageway with a roof but not walls. Also if the garage shares a wall with the house, that is attached even if you have to go outside to reach the garage. (I grew up in one of those).
It matters because subpanel rules are quite different for detached (local main breaker / Rule of Six comes into play), and SER is not allowed outdoors. If a garage meets the NEC meaning of "detached" I don't see any way to get there without going outdoors. Conduit outdoors is outdoors, presumed to be full of water.
Will have service disconnect on sub since detached garage and >6 circuits. Service disconnect is 100amp
Make sure the panel has loads and loads of breaker spaces. Breaker spaces is the greatest parameter of concern in panel sizing, because people pathologically go into "penny-saver mode" when sizing the panel. This is foolhardy. They have no grasp of just how much stuff an 80A panel can support, and the "cost delta" to an ample-sized panel is the price of a pizza. 24-30 spaces is not excessive, the cost is trivial on total project cost.
Often these "ample" panels will only come with 200A main breakers, which is fine.
But a not-actually-detached garage does not require a main breaker/disconnect at all.