Welcome back, Ashrah!
Everything everybody said is right. Also lotsa stuff they didn’t say.
Let me throw in some assorted points.
- Designed path
What Blizzard expects you to do is
- level quickly to level 50,
- then level through the long, linear quest chain in Shadowlands to 60
- then gear up through World Quests (like dailies but different) and Daily Quests in the new zone Korthia
- progress in dungeons, raids, PvP to taste.
At 60, you will not need anything from a previous expansion, so none of the previous expansion is required.
The advantage of this path is that you will then be up with all the other players, doing what everyone else is doing.
The bad news is that
- You’re gonna feel like Franz Kafka designed the opening quest paths at 60
- Shadowlands end game is unusually shallow for questing and world content
- Completer - Your other choices
There is a vast amount of content in Cataclysm, Mists, Warlords, Legion and Battle for Azeroth content you have never seen.
Alts are VASTLY faster and easier to level than you remember. You can level an alt from 10 to 50 very neatly by questing through an expansion. So, for example, pick Mists, quest through its zones. You will get through nearly all of it at the time you hit 50.
If you really want to see lots of the content you have missed, the ideal way is to make one alt for each of the previous expansions. And, of course, at the end you will have a stable of 50s from which you can choose to go forward to level 60.
You can hop between them, levelling each a bit at a time. However, you then get into feeling that nobody is making progress. So, if doing this, it’s probably best to play one for a chunk of levels at a time, enough to feel you have made progress, before switching.
The advantage of this path is that you can tour the whole game at your own pace (and make huge progress on your Safari pet collection achievements (and btw the Snowy Owl has just come into season in Winterspring, so be sure to capture one before Winter ends.)
Or, ofc, you can send one character on to 60 while leisurely levelling alts throug the expansions. All up to how you feel.
- Resources
Websites to rely on:
Wowhead for information about quests, items, all that.
Icy Veins for information about how to play your class and spec
Curseforge for addons. And if you are doing addons, check out WowUp as an addon updater.
Pets are well covered, Warcraft Pets for general information, and, later, when you are battling level 25s, Xu-Fu’s has all the strategies.
And of course, if it’s in WoW, it’s on YouTube somewhere!
- Battle Pets
I see you have actually gathered a significant collection of wild pets already as you have levelled. Good work, there!
I have a five-part guide to making progress in pets in the Pet Battles forum starting here. It may be a bit too focused on progress for your current intentions, but there is lots of help and information available in general, and I’m always happy to help out a new battler. There’s a very friendly Discord, where we’re all willing - and maybe too eager 
- to dish out advice and spare tradable pets.
Do please ask again about anything you might think of!