How to choose a password that I have to remember for a long time but do not use a lot

Likely the best option in this kind of scenario is to record the password/passphrase in a physically secure location (e.g. bank vault, safe deposit box).

Relying on human memory to record this kind of information for 10+ years is an extremely bad idea. For example the person who knows the passphrase leaves the comapny/gets hit by a bus/forgets it.

Writing it down and storing it in (preferably 2+) secure location avoids this kind of issue.

I use the following system:

Hash a string of these three parts

1. A fixed, long and impossible to guess prefix (I diceware'd a 10-word phrase)
2. The current date to the precision that you can easily recall/depending on the frequency you change it (e.g. use year only if changed once every few years in case you forget the month)
3. An intuitive and succinct description of the purpose of the password (e.g. domain name of the website or a single word like "DR")

Use this pattern to generate all your passwords so you'll never forget the fixed prefix. Parts 2 and 3 prevent collision so if any password is revealed, you don't have to remember a new prefix (and then forget the old one by mistake).