What's the notification system that has to be in place for this? How do I make sure I go through every single block, 6 below the tip. Do I need some kind of record to keep track of the blocks that I've processed?

Also, what if I want my users to know that their transaction is seen by my system all the way from mempool to 6 confirmations? Should I be catching transactions in any way?
Maybe walletnotify or zmq?

Having a withdraw only core means to have a core with a few addresses that I can top up at any time and simply use sendtoaddress rpc? How can I possibly improve the privacy of this? Any way to make withdrawal transactions more anonymous for customer's security?

Are there any rules of thumb when working with walletnotify and blocknotify? Having an express instance running to process curls from it feels reeeeally awkward...

Any recommendations on how to control the bitcoin core? Should there be more than one core running simultaneously? Should there be a program that bitcoind can be run behind? Any research or guides on this topic?