Feeder Wire Size for 100A ADU Subpanel

Hi all! Would appreciate any and all advice for determining the correct feeder wire gauge for a subpanel for an ADU. Not an electrician so I apologize if I butcher any of the verbiage or misuse “feeder” vs “service”. Not doing the labor either, but just staying prepared and cautious after our previous electrical contractor tried to convince us a #6 aluminum was rated for up to 100A.

For context, we are converting an existing 600sqft detached garage to an ADU. Currently have a 200A service to the whole house. There is an existing panel 50’ away from the garage (coming directly from a 100A breaker off the main distribution) that is no longer being used (used to be for old mini split ac condensers since removed). The plan is to place a 100A breaker in the panel and run a 50’ length of wire to power the subpanel of the ADU. From the main to the panel, there is already a #2 copper service line running about 80ft (https://imgur.com/a/JnjT733). We will use schedule 40 PVC as conduit above ground (mounted along the side of the house) for the 50’ stretch from panel to subpanel. We live in a tropical climate.

It’s hard to understand the NEC tables based on circumstances. What size feeder wire can we use for the 100A ADU subpanel for the 50’ length for either aluminum or copper? It appears NEC would allow for #2 aluminum, based on all the tables regarding dwellings? Other Reddit threads mention #2 aluminum is only rated for up to 90A? Could a #4 copper wire work as well? Thank you in advance.

Location: USA