Probably you should read up or watch some youtube videos on accounting. (H)Ledger is a great tool which allows you to find your solutions to your problems.

If you want an example for standardized structures of accounts take a look at the german "Kontenrahmen" https://de.wikipedia.org/wiki/Kontenrahmen.

Cash can be dealt with in many ways. Some only track the ATM transactions.

Interests are just transactions, too.

HBCI/FinTS is a great way to keep track with your actual bank account.

Usually tax reports only need a few numbers. Write up some script that prints these numbers for your ledger account.

(H)Ledger does not support recurring transactions. But there are a lot of unix tools that do recurring things (cron, systemd-timers).

I would keep ledger files for private and businesspurposes seperate.

Keep in mind, that (H)Ledger is plaintext. Automation is simple. If you want to track expenses on the go, my advise would be to write a Telegram bot.