It's always been a worry of mine that I might have an issue with renewing some of the domains I really rely on, and that sorting out the problem might take long

It's always been a worry of mine that I might have an issue with renewing some of the domains I really rely on, and that sorting out the problem might take longer than the amount of time I have left before it expires. So one year, whenever a domain came up for renewal, I bought a two year renewal on it. And since then, I renew all my domains at the same time every year (first week of January, but you could do it any time) when there's between 1 and 2 years left on all of them. If there's ever a problem dealing with the renewal, I've got plenty of time to sort it out. Yes, the registrar gets to keep (on average) 18 months worth of registration fees on account, as it were, but for me it's worth the peace of mind.