how to block incoming connections by their IP address

> [...] For my FTP server, [...] I will have to consider using an odd
> port for that too. [...]

   If you're sufficiently annoyed by the break-in attempts.  I run an
exposed FTP server, and I'd estimate that I typically see a few serious
password-guessing attempts per month, but seldom repeated attacks from
one remote address.

> [...] It just bothers me that my CPU and OS have to handle these
> constant interrupts, [...]

   I don't see much of a resource drain.  Or "constant" anything.

> [...] I would think that it should not be that hard for the firmware
> to store a list of IP's (defined by me in the Security section of the
> interface) to be ignored. [...]

   You could be right, but Netgear firmware for consumer-grade routers
is chronically buggy, so it might make very little sense to (or for)
Netgear to invest resources in a feature which will be used by such a
small fraction of its customers, when it has so many more important
problems to solve.  Like deciding which existing features to remove, in
order to make room for new, revenue-generating features.  One of which
this wouldn't be.

   Beside which, I wouldn't bet on their ability to implement it
correctly.

   Yet another potential waste of time and effort:

      https://community.netgear.com/t5/x/idb-p/idea-exchange-for-home