Yes.

But: if you use default settings, then probably not all keys are hardened. And the last keys are probably non-hardened. You can compare it with my example. You should get the same results, even if no wallet is loaded:
Which means, that if you put your results from "listdescriptors" here, then you can check, if it works for your keys. For deriving keys, you need only "tpub" (testnet master public keys), because "tprv" (testnet master private keys) are needed only for spending.

Well, I always manually pick all inputs and outputs. Also because Core wallet usually gives too huge fee estimates, or pick UTXOs differently, than I want. But: in case of automating things, I would probably batch as many things, as possible, which means collecting all withdrawal requests, and handling them every sometimes (once per 6 hours, once per day, or something similar). It is also possible to offer faster processing, but then, fees will naturally be higher, because then, transactions are not batched, if you send coins to a single user.

I remember, when as a customer, I paid for example only 80 satoshis, when my withdrawal was batched in a group of hundreds of users, and when mempools were below 4vMB, and accepted 1 sat/vB fees. As a single user, if I would handle it manually, I would probably pay at least 110 satoshis, if not more. So: batching is the way to go, if customers are not in a hurry.