When creating/editing membership. End date is limited to 2026 (you cannot choose it in pop-up calendar and if you try to write the date yourself(exceeding the limit) php will not let you do it. which is not enough. The end date limit must be at least 50 years more. civicrm/admin/setting/preferences/date?reset=1 is not the page that is needed. Where would I be looking for things like that? Can I configure it through the website itself, or do I have to go look at the source files?
I just replicated this problem - but I was able to successfully change the end date limit on the "Date Preferences" page that you said wasn't working. Under Administer menu » Customize Data and Screens » Date Preferences, you can change the "activityTime" values and this will update the Membership end date.
The language on the page is a little vague:
Date for activities including contributions: receive, receipt, cancel. membership: join, start, renew. case: start, end.
IMO it should read:
Date for activities. Also the date for contributions (receive date, receipt date, cancel date), memberships (join date, start date, end date), and cases (start date, end date).