The type of cable you linked to is SE cable covered by Article 338 of the National Electrical Code. It connot be used underground with or without a raceway. Here is the pertinent code language.
338.12 Uses Not Permitted.
(A) Service-Entrance Cable. Service-entrance cable (SE) shall not be used under the following conditions or in the following locations:
(1) Where subject to physical damage unless protected in accordance with 230.50(B)
(2) Underground with or without a raceway
Notice item 2.
You could use several other types of insulation in a raceway, including THHN, XHHW, XHHW-2. USE is for direct burial and can be used without a raceway except where exposed to damage so you need to sleeve it in PVC or metal raceway above ground.
Aluminum also needs an anti-oxidation compound such as NOALOX or ALNOX, there are many brands.
Rigid or IMC conduit can be buried only 6" deep. Table 300.5
SE cable can be used as a feeder for interior wiring so if you kept the feeder inside the house and garage without going outside you could just protect it with PVC in the garage and be good. Or you could run the cable outside just not underground.
You will need a single 100 amp 2 pole breaker to feed the sub panel. 2 -50 amp breakers is not acceptable by the Code.
A separate disconnect is not required. The panel in the garage is not a motor so the "within-sight rule does not apply". The breaker in the main panel can act as a disconnect. You actually don't even need a main breaker in the panel if you don't want one. You can buy a Main Lug Only panel if you wish. The main only panel you linked will do just fine without a main.
Good luck and stay safe!
Sch 40 is fine, this time at least
Since this conduit run is inside a wall, its generally not subject to physical damage, which means Schedule 40 PVC is fine for this job. I'd simply bring it out of the back of the box with a LB body, by the way, and then it can just pipe straight into the side of the indoor subpanel, or into the bottom or top with a LL or LR body if that is more convenient.
As to other means? PVC is really your best choice here, unless you want to go metallic instead, which puts you in EMT-land. (Rigid is a bigger pain to deal with due to threads, and SE cable is going to be harder to replace later if you want more power in the garage.) I'd just size it generously -- 2.5" is a good size for this.
Wire derating is not needed here
Insulation isn't an issue when it comes to derating for wire in conduit as long as the ambient temperature isn't roastingly hot (i.e. not greater than 30°C/86°F).
Don't forget the grounds
Since you're providing a feeder to a detached outbuilding, you'll need to both run an equipment grounding conductor back to the source of the feeder (it can be 6AWG copper) to provide a fault path, and run some more 6AWG copper back to a grounding electrode (ground rod) for the outbuilding to deal with lightning-type problems. Don't forget to pull the bonding screw in the disconnect while you're at it!
Best Answer
You haven't stated much about the subpanel in the pool building, and it would be nice to have 100A in the garage given the plans you have, and the plans automakers have for future vehicles.
So I would simply extend the pool circuit as 100A. You can do that with lugging only - you won't need to add a bulky circuit breaker in the potentially small pool panel, since the 100A main panel breaker feeding the pool will also protect the run to the garage, being all 100A.
100A feeder is typically #1 Aluminum, unless you own a copper mine, in which case #3 copper will suffice. I'm not a fan of copper because most splice lugs and subpanel lugs are zinc plated aluminum, and why introduce dissimilar metals and pay more for the privilege?
On the garage panel, I see a need for at least 14 spaces (assuming you graduate to 240V compressor, table saw and welder, plus 4 120V circuits plus two 240V EV chargers. The worst mistake you can make is short yourself on breaker spaces, so think big - like 30 spaces. You do need a shutoff switch in the garage subpanel, and usually people just select main-breaker panels but the amp rating doesn't matter. 30 space panels typically have 150-200A main breakers, which is fine.
Never rely on 'double-stuff' breakers to solve full-panel problems. Most breakers these days need to be AFCI and/or GFCI, and those are simply not available in double-stuff.
It would sure be nice if the pool panel had "thru lugs" for onward connection to the garage. Think about changing the pool panel for one with thru lugs.