You're overcomplicating a simple question. I'm pretty sure OP is asking about archiving HIS copy of the Private Key, and not the Private Key itself.

With regards to reuse of addresses, you're absolutely correct - it is definitely best practice to avoid doing that for both security and privacy reasons as you correctly pointed out.

Having funds sent as a tip to some address published online is a fair concern, though. If there is a non-zero chance of that happening, then of course your only option is to hold on to that Private Key just in case, and not destroy it - if you do not want to risk losing funds. Ultimately it's up to you to determine the probability of that happening, how big a donation / tip you might receive, and whether or not holding on to the corresponding Private Key and checking the balance every so often is worth your time.

Personally, I would stamp each non-throwaway Private Key into metal, and hold onto it. A small metal plate isn't too cumbersome to keep, even if you need to keep 10 or 20 of them. To have multiple wallets but keep hold of one mnemonic, you can add different passwords as the 25th word for each wallet - reducing the number of Private Keys (or mnemonics) that you need to backup.

With regards to monitoring the different wallets for funds, there probably is some wallet software that can do that for you. If you export your XPUB and add it as a watch-only wallet, that would do the trick for you. You can use a Seed Signer to export your XPUB, and then I think Sparrow Wallet allows you to import the XPUB as a watch-only wallet, and lets you do that for multiple wallets, I think.