Well turtling for ages is likely a bad idea in the first place. You need to be out levelling your hero and getting more troops and crafting gear and recipes right from the start (lots of quests e.g at inns and villages can get you extra troops early on, or you can just recruit some soldiers).
You need to find sites for new cities and where other shards are fast and expand to them as soon as possible
I don't enjoy playing the campaign on Insane because everything having such high hit points makes every battle take forever to fight. It's not so much that it's difficult as slow and so dull.
If you want to play something different from the original campaign then i'd suggest playing a non-campaign game (just click 'New game') and play on a random map. It gives you various difficulty options, other than hit points.
I usually customise whatever Sovereign i choose to have the Vitality spell (basically a double move for one army in an emergency) and the Ranger starting hero. The Ranger isn't the best in combat (Varda and the Cleric are by far better fighters), but he can fight from a range and gets the Hawk skill that gives you a hawk that can collect crafting ingredients and recipes and even go to inns you can't get to early on. If you get the 'Return Home' scrolls recipe (which is quite common), those troops can even be sent back to your capital with them.
Try to end all the hawk's moves over either the sea or a mountain so it doesn't get caught and killed.
The Ranger also gets a skill "hunter and gatherer" that gets you extra crafting item loot from every battle
Choose the skills that get you a second hero and bigger army size soonest - but save the game before you click the 'New Hero' skill when your sovereign levels up, in case you get one you don't want. Then you can reload. Tandis the warrior is pretty good because he can get 'Protect' and 'Inspire' abilities that raise the Defence and Dodge skills of the rest of the army, so an army with him and the ranger is pretty strong. If you prefer an offensive hero, Varda is by far the best at attacking.
Combining the Ranger with the Tinkerer Sovereign can work because they each get skills that get you extra loot. Getting a few armour pieces, a Hunter's bow and some hunter's swords early on can make a big difference. If you enchant the swords, save the game, exit the game and then restart it and reload, then any soldier or rider or spearman or rider units that you had recruited before you do that can equip the hunter's swords - and they only need to be level 3 to equip them. The hunter's bow can be equipped at Level 1 and is a big improvement on the Ranger's starting bow - he can do some real damage with the hunter's bow.
The spider he starts with works well in combination too, webbing enemies so he has time to shoot them.
The bear doesn't have enough hits or defence to be much use except in support, unless you put some leather armour on it, so i usually use swordsmen (plus any spearmen i can get from inn quests) - with whatever armour i can find or craft shared between them , the ranger and the bear. Try to hang back and form a line with the swordsmen - advancing to attack first is usually a mistake unless the enemy have lots of archers and few melee troops.
Tinkerer sovereign plus ranger also lets you summon the Vorpal Longbow in combat from the start of the game, for just 4 mana, effectively giving you two archers. The Commander is probably still a better sovereign, but Tinkerer with ranger hero is interesting to play - and you can craft some amazing Legendary weapons, armour and jewellery with him late in the game