About LTS - it is all about support periods. If you spend a month setting a a server (and when you are new, it could be a year to setup, easily, then you don't want support to end without 3+ yrs of support still remaining.
When I was new to Linux, I wanted to know everything too. IT IS NOT POSSIBLE, so get that idea out of your mind. I've been doing Unix/Linux since around 1993 and I guess I know 10%. Things are always changing so answers usually have history behind them.
If you want to learn Linux as a server + light developer, then start with the basics that end users need to know. https://www.linuxcommand.org/tlcl.php . Work through the first 12 chapters of that to get the basic understanding. Around pg 250, things that seem disconnected should start to click together and you'll gain insights that just aren't possible memorizing commands. That alone will take 6-12 weeks, depending on your commitment. The goal is true knowledge, not finishing each chapter and all the exercises. Only you can know if you truly understand users, groups, permissions. Even after getting an "A" in a Unix course because I could memorize those things, the elegance of Unix permissions wasn't clear until I sat down and played with 3 different userids, mixing and matching groups, then seeing what every single possible permission did on the files AND on the directories. I literally did this:
Code:
chmod 007 file
chmod 006 file
chmod 005 file
chmod 004 file
chmod 003 file
chmod 002 file
chmod 001 file
chmod 000 file
and between each of those changes, saw what the owner, group member and outsider could see and do with the file. Then I moved onto
Code:
chmod 100 file
chmod 200 file
chmod 300 file
....
chmod 777 file
You need time to "sleep on it." Lots of time as you learn more and add the new points of knowledge into your overall Unix understanding.
For example, 711 is very useful for programs and directories in ways not intuitively obvious, but not ok for interpreted scripts.
Also learn what is meant when people say that in Unix "everything is a file".
These things are base knowledge and feed into EVERYTHING ELSE on the system.
As for security, it is a process, not a program. In general, the larger a program or set of installed programs are on any computer, the less secure it will be. Alpine is small, so more secure than Ubuntu, but there are many assumptions around that statement - like "all other things being equal". Any computing device can be non-secure, if the admin and users make foolish choices. Out of the box, alpine is more secure because it has nothing enabled. If you want something to work beyond a minimal shell, then you'll need to set it up. OTOH, post install Ubuntu Server does lots of things, some over the internet. Is that more or less secure? It certainly can be handy. That's also certain.
I've decided that Ubuntu Server is secure enough by my needs, with a few caveats. Ask again in 3 yrs and we can chat deeply about that. That this point, you don't have the background and I don't have the time to type it all.
If you want to learn how to setup a Linux system, get comfortable with an Ubuntu desktop flavor suitable for your hardware. Run that for 3 months. Everything you can do on a "server" is possible on desktops too. Getting to the point faster is usually a good thing so people aren't overwhelmed with things that are nearly automatic on the desktop releases. Do you really want to struggle with setting up network access via text config files that nobody except Canonical uses? Is that an efficient use of your time?
Next, install something that shows more of the details like Arch or Slackware. Run one of those for 3 months.
Then come back and load Ubuntu Server. Run that for 3 months. Or Debian. Your choice. They are different, but not completely different.
Then decide if you might want to switch to a RHEL-based server or a SuSE-based server to get their take on it. They come with SELinux installed, which can be a pain, but has lots of features not in Debian/Ubuntu.
For end-users, Linux is Linux is Linux. Over 90% is the same. It is the administrative aspects that are different, but the underlying system is the same. Alpine, debian, ubuntu, rhel, SuSE, arch, gentoo ... are 90% the same. They all get their kernels from kernel.org (that Linus runs). Of course, kernel versions can matter greatly, depending on your hardware and specific uses. For a while, ZFS didn't work with the v6.x kernel. If you don't use ZFS, it doesn't matter, but if you do ... you'd either know this or be bitten.
Oh ... learn about volume managers and file systems sometime around 8 months of daily use. Beginners aren't used to thinking that there are 10 popular, but different, file systems. If you don't know anything, choose ext4 on Debian/Ubuntu. Some other distros will push some other file systems, which is great. Just beware that data loss can happen in some non-standard uses.
Did I mention backups? You need those.
You may notice I didn't say much about being lean, since that's really low on the things someone new should worry about. There are bigger issues to solve.
