I’m planning to upgrade the service in a detached garage, which is currently fed by a single 15amp circuit, and I wanted to confirm that I have my circuit sizing correct. The existing circuit runs through ¾” IMC buried under the driveway, which I’d like to re-use to run a new feeder circuit to a sub-panel in the garage. My read on the NEC is that per Table C.1, I can have a maximum number of 4 conductors in the 3/4" conduit for 6AWG THWN-2, Table 310.15(B).16, says I can run a maximum of 75amps on 6AWG copper THWN-2, and 310.15(B) states that I do not need to de-rate for 3 or fewer conductors. The rest of the circuit (main load center to buried conduit, then from buried conduit to the sub panel) will be run though 1” PVC conduit. Based on this, I believe I should be able to run a 70 amp breaker for this circuit. Am I correct on the sizing, or am I missing anything?
You're missing the termination temperature limits from 110.14(C)(1)
You are correct that you are missing something. In particular, while THWN-2 wire can handle 90°C without an issue, that doesn't mean that the things hooked to it will be happy at that temperature. In particular, since we're dealing with a feeder here, distribution equipment, such as circuit breakers and loadcenters, is only rated for the use of 60°C or 75°C wire because the thermal elements in the circuit breakers themselves can be thrown off by excessive heat from a wire running at a higher temperature. This requirement is captured in NEC 110.14(C)(1):
(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).
(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60°C (140°F).
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.
(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75°C (167°F)
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors
In particular, since modern breakers and loadcenters are basically universally marked 60°C/75°C, we can use the 75°C temperature rating for ampacity calculations, but can only use the 90°C column of the ampacity chart as a basis for derating. With this in mind, we look at the 75°C column in Table 310.16 and get a rating of 65A for your 6AWG copper THHNs. This means you can either use a 60A breaker and a 10AWG ground wire (bare or green THHN), or a 70A breaker and an 8AWG ground wire, with the latter permitted by the "round-up" rule in NEC 240.4(B).
If you really need 75A at the garage, you could use UL 1953 listed power distribution blocks in pull boxes at each end of the IMC run (one outside or inside the garage, one in the house) to transition from the 6AWG THHN inside the ¾" IMC to 4AWG THHN in the 1" conduit you were planning to use for the interior runs. This would require you to use an 8AWG equipment grounding conductor alongside that 6AWG THHN instead of the 10AWG suggested above, but is permitted by NEC 110.14(C)(2), as documented in this Square-D document
Note that unless you have "straight shot" runs from the underground conduit to the panels at both ends, it may be more economical to use 1" ENT ("smurf tube") instead of the 1" rigid PVC you were originally figuring on. Even when using ENT, you still have to obey the 360° limit on bends between conduit pull points, though!
GO BIG OR GO HOME
However, regardless of whether you run the underground wire at 75°C or 90°C, you should not let its ampacity be a constraint on the size of the panel you put in at the garage! In fact, a 125A, 24-space or 30-space, main breaker (to provide a main disconnect for the garage) panel would not be at all out of place here, and nothing stops you from putting in something larger yet, such as a 40-space or 42-space panel. Regardless of what you get, though, you'll need to fit it with separate grounding bar kits if it doesn't have factory-fitted grounding bars. Furthermore, you'll need to ensure that the neutral/ground bonding screw or strap has been removed from the panel so that it's configured correctly for use as a subpanel.
Finally, you'll need to take some 6AWG or 8AWG copper wire and run it from the subpanel grounding bars through something like an Arlington GC50 to get it out of the subpanel cabinet, then take it off to a pair of 8' grounding rods driven at least 6-8' apart. This provides your garage with a path for wayward natural electricity, such as lightning-induced voltages, to return to Mother Nature, while the grounding wire in your 4-wire feeder provides a return path for errant utility electricity to get back to the utility service via the neutral-ground bond in the main service panel.