For a while I used CentOS 7 on all of those small VMs, because it got security updates for a really long time. With minimal risk of breaking things on updates.
I've upgraded Debian stable (both pure and with some cherry-picked backports) and Ubuntu (non-LTS and LTS) systems in place and rarely broken anything, for years and years. When stuff has broken it's been a quick google and then slapping myself for not having read the upgrade guide.
I do generally wait about 2-3 weeks before upgrading, giving time for them to catch stuff that was missed until the great masses were set loose on it.
I've had issues with Ubuntu/Debian upgrades more than once. Some third party binaries breaking with the update. Or some specific config tweaks that break, because the structure of /etc changed too much.
For some small VM with a specific purpose I prefer a distribution that changes as little as possible for as long as possible. Less work, more uptime.
My guess would be that fastfetch probably reports actual memory usage while btop probably reports the total usage of all processes. The former is probably higher because of things like filesystem caching