For example, the XPUB is stored only on the BTCPay API server in watch-only mode, which means that an infinite number of Bitcoin addresses can be generated for receiving tips while keeping everything secure.

The question is: if this is for tips (aka small amounts), why have to be onchain (for fuck sake!). BTCpay also support LN invoices, why not use that API?

Geeze guys, stop building payment solutions for onchain.
Onchain will be exclusively for opening / closing LN channels. All other payments will be only through LN.

You have been warned.