I have upgrade anxiety because I believe it won’t go as smoothly, and I’ll end up spending a weekend or two or three or god knows what.
Partially, it’s coming from not being able to understand what the install script does exactly.
Is there a way to install a new version on another box/VPS, etc, and then just switch over?
I’ve been running MIAB for a few years now, and never had trouble with upgrading to another version. (I run a reasonably standard MIAB box but self-hosted.)
I have kept a record of the (minor) tweaks I made, so I can check/recreate them after upgrades.
If you’ve got the current backup copied to another machine, the restore process is fine.Should the worst come to the worst, you wouldn’t be down for long 
I too have been running MIAB for several years now. I have found the upgrade script to be very reliable and of the past upgrade or two, I have had zero issues.
Previously, the one issue I had to manually correct was to update my IPV6 address in nsd.conf. For some reason, which I believe was addressed in forum posts, the script wasn’t picking up my IPV6 address. Again, I understand this to be corrected now.
Yes, I had to do this to move from Ubuntu 18 to Ubuntu 20. Most cloud providers will allow you to spin up a new VPS, set it up, and then swap the IP. You can then restore your MAIB settings/accounts from the backup file.
Mine is set up on Linode which allows for an easy swap of the IPv4 address. The IPv6 address is different on a new VPS due to how their assignments work. In my case, my IPv6 is tied to an added range (better since their IPv6 addresses fall in ranges that Spamhaus doesn’t like) and I had them move that over as well.
So, set up your new VPS, move IP’s if you can, set up MAIB, and then restore from the backup. If you do need to make changes to your IP, update your DNS records accordingly, and wait for propagation. Some records update quickly, some can take a day or so to wait for TTL to expire.
Your mileage may vary depending on the needs of your mail server. Mine is just for my personal domain so I wasn’t overly concerned since I don’t use it for anything critical.
Most VPS providers allow you to take a snapshot of an instance using their Web interface. I suggest you do this for your mailinabox VPS, then do the upgrade. If anything goes wrong and you can’t easily fix it, take note of the error messages and then simply restore the snapshot. That will give you a functioning server while you figure out what went wrong and how to fix it for the next upgrade attempt.