Thanks Vince. I didn't include error messages because my question was (and is) whether there are other options for a small CPU I can use to replace the Pi/Raspbian. I appreciate your letting me know that you have confirmed Ubuntu can work on the Pi 4, I may try that.
I see the differences between Ubuntu (which I am getting pretty familiar with) and Raspbian (which has some annoying differences in how users are implemented for example, plus the various issues below) as distracting from my main goals with WeeWx.
With regard to the SSH passwordless login issue, I have been running into "Putty key format is too new" when using the keys created by Puttygen in Putty to log into a remote server. So this indicates to me that Puttygen is downrev on the Pi/RaspbiOS. I'm sure there is a way to make this work but I haven't found it. There are multiple options for generating keys. The ones I've tried don't work on Raspbian thus far. I have updated all the packages and gone through multiple reinstalls to make sure I wasn't tripping on some small detail.
I don't see how firewalls or sudo play into these problems.
I'm hearing you that you haven't had these problems with Pi. You are clearly much more of an expert and are working from solid install/config procedures that I don't have, since the (multiple) ones I've tried are not getting me through to the result.
As for MariaDB, I have used it for years and the problems I'm running into with RaspiOS are ones I haven't seen on Ubuntu. Where I am stuck on that now is when running mysql_secure_installation after the basic MariaDB install, the root password is already set somewhere during the (very simple) database install, so it is impossible to set the root password (it's not blank as it is supposed to be, coming out of the install). This just seems to me like a bug. My motivation to use MariaDB is that I'm used to it, I've used sqllite before and it does not support the ability to do remote queries, which I'd like to do from my NodeJS web application to gather specific data from WeeWx data to incorporate into my app. If this were the only issue I'd probably just fall back to sqllite for now.
I built an entire NodeJS web application for Pi that included MariaDB a few years ago, so I've seen MariaDB run on Pi successfully. This issue is new.
Anyway, thanks again. I was wondering if the quality of the Pi/Raspbian has fallen behind but you are vouching for it so that is good input. I'll be thinking about my next move.
What's frustrating is that I've had WeeWx running on a Pi 3 and pushing data via FTP for years to my hosted site, until the hardware gave out. I didn't think a reinstall on a new Pi 4 would surface all these issues I hadn't seen in my previous experiences with the Pi.