Make it easy on yourself and just use Tor. Clearing everything is a matter of clicking on "new tor circuit for site".

Some sites throw annoying captchas your way though.

You're doing voodoo. It's not you that's being rate limited, it's xcancel itself that is being rate limited. If you wait a few moments and reload, things will likely work.

Although now I should say that this was the truth yesterday, but now twitter seems to be actively killing the credentials that nitter instances are using, and they're returning rate limit messages permanently. My guess is that this is a function of how many credentials/tokens a nitter instance was cycling through to get content x the amount of traffic that nitter instance gets. They probably just set the rate-limiting bans longer for everybody (or steadily increase them on each offense), instead of trying to find the specific offending credentials.

edit: xcancel officially gone, so none of this matters.