Cities Skylines can provide among the best city sim experiences to be had if you're willing to invest a day (or two) modding it to within inches of its life.

That and 32gb of RAM

Is there a list somewhere of "Essential" mods?

The CitiesSkylines subreddit has a list that contains (IMO) a good number of useful and essential mods.

Word of warning: if you're one of those people that likes to install a lot of mods keeping everything up to date and non-conflicting can be an pain the rear. There is, of course, a mod to keep track of which mods are out of date or could cause issues, but that's really a band-aid on top of a larger problem. The game is old enough now that many popular mod authors have long been abandoned their projects or otherwise moved on so you end up with things like "MyPopular Mod" and "MyPopular Mod UPTODATE" or "MyPopular Mod FORKED" or "Use this instead of MyPopular Mod".

If I take a few weeks / months between plays this usually ends up with me having to take 30 minutes to an hour to get the mods up to date and tested. Not a huge amount of time, but when your gaming time is limited it's something to take into account.

You could just... not update every time you play ?

They still add updates to the game once or twice a year, so you usually want some of the new features. That can also lead to the issue of when you finally do decide to update 3 years later or whatever, now it's a 4 hour ordeal to fix instead of 30 minutes. There's no really perfect solution, but it's worth it if you love city builders.

Steam updates the game and mods automatically. It can be overridden but that's a pain in the behind.

Can it ? In my experience, once Steam is aware that there is an update, you cannot even start the game before updating, even in offline mode... but that was some years ago, maybe this behaviour has changed ?

(Don't use Steam.)

Steam has the option to let you pin a specific version of the game if the publisher supports it, but Cities Skylines does not.

Yes, that is an annoyance. I think that Steam should put more pressure on developers to make old versions of their games available. I am glad that the software support is there (even if the UI is buried; it was originally intended to support beta–testing programs, complete with passwords to unlock the beta releases), and that developers sometimes use it, but I wish it was a formal requirement for publishing on Steam.

If you don't mind video format, I like this person's guide. He also has a 2nd video with essential mods for "advanced" players.

https://youtu.be/Nr8Wlib4jqE

He also has some fun youtube series' where he builds up a region by following a "story" he makes up (or sometimes uses viewer polls). So rather than building a super optimized city, it gets a very natural layout as he pretends to play out rivalries between developers, mayoral or policy changes, etc.