I've been downloading SSL certificates from my domain provider, using cat to join them together to make the fullchain.pem, uploading them to the server, and myself adding a 90 day calendar reminder. Every time I did this I'd think I should find out about this Certbot thing.
Well, I finally got around to it, and it was one of those jobs which turns out to be so easy you wish you'd done it ages ago.
The install was simple (I'm using nginx/ubuntu).
It scans up your server conf files to see which sites are being served, asks you a couple of questions, obtains the Let's Encrypt certificate for them, installs it, updates your conf files to use it, and sets up a cron job to check if it's time to renew the certificate, which it will also do auto-magically.
I was so pleased with it I made a donation to the EFF for it, then I started to think about how amazingly useful Let's Encrypt is, and gave them one too. It's just a really good time to be in this hobby.
I highly recommend Certbot. If you've been putting this off, or only just hearing about it, make some time for it.
Wait until you set up cert-manager to issue both Let's Encrypt certificates, as well as generating your own CA and issuing certs from your own CA where you can set the validity however want.