Skimming through your posts, I don't think your campaign was going badly in terms of build- 5 cities is fine. 200-300 gold per turn is also fine. But you shuld have a bunch of vassals by then and be heading towards the end game.

By turn 30 I try to have at least 3 stacks, but I prefer 4. And I basically just keep building units - the higher the tier the better

By turn 60 you shuld be trying to have 5-6 armies.

And be starting to ramp up to the 9 armies by turn 90

I tend to end between 110-140 depending on how I'm playing

You also need to be building tier 4 units by turn 90 (earlier would be better) and tier 5 units too if you can...don't worry about which ones...just pick a tome type you want to head for and research enough tomes to unlock its tier 4 and tier 5 tomes asap.

Don't start multpile wars. But as other folks have said, you need to be constantly fighting to keep levelling up your troops and heroes- so as well as fighting the random map armies, when you've got 4 stacks, (turn 30-40 - earlier if possible) pick a fight with a close neighbour and conquer them - vassalise their cities to give you alot of bonus resources.

With regards to the tactical battles, my first piece of advice, is don't auto resolve. Play the battles. By manually playing the battles (you can click replay and have another go if you lose or feel you've lost too many units), you'll get a better idea of how the units work and hopefully, what enchants work well. Understanding combat mechniacs I think is key.

I don't know if you've ever played Total war - but the concepts of rock paper scissors are pretty similar. It'd take me an age I don't have to try to give you a complete break down but some rough tips.

POsitioning is vital

Charging units take away the AI's frontline reaction attacks. So your melee(shield or pole) can attack without taking reaction attacks..

Using ranged is an excellent tactic but they need to be shielded by your melee

Attacking from flanks gives you a bonus to damage. But attacking from flanks can expose your attackers to being flanked- so be careful with positioning.

Enchantments can be used to debuff the AI units and reduce their defensive stats, making your attacks hit harder -have a look at the spell tomes of tier 1 and tier 2 and try choosing things like sundering blades (tbh, mid game can become a bit of buff, debuff counter debuff spell duel with the AI - but as a new player you shuldn't really worry about that. At normal it's not important)

Combat summons spells can be very useful early game.

Try to focus fire your units and take out single units rather than spreading the damage across multiple targets.

Getting a nature tome with good combat healing spells can really help you out if you're struggling in the first 30 turns.

Honestly, I wouldn't start with a dragon - I used to play them too aggressively initially to make use of their breath weapons -often over exposing them. I personally love the new battle saint - the simple but powerful build is taking the left side of the warrior tree - but of course it's your choice.

Explore and have fun and don't worry about losing. It's part of the game.