Thus, by the middle of the game, it becomes difficult to find and combine artifacts, as 2/3 of them are with the escaped heroes, and who knows when they will appear in the Tavern again...

You can simply choose a hero in the tavern instead of waiting for one to appear there.
Here is a link to the HD MOD program, which makes the game easier - widescreen, the ability to choose any hero (from those allowed and available at the moment), control over the remaining movement points, and many other convenient and useful game features (the gameplay and structure of the game do not change).

https://sites.google.com/site/heroes3hd/description3

By the way, I read somewhere that the maximum you can get is +3 to Morale and +3 to Luck. It seems that you can get more with the help of artifacts and various events, such as visiting the Fountain of Luck, etc., but you shouldn't expect more than +30% chance of a double turn / damage. Or does +3 not equal +30%? How is this calculated?

Yes, the maximum is +3, which gives a 12.5% chance of luck/morale. Below the link, download the FizMiG reference guide - it contains answers to many of your questions.

http://homm3world.narod.ru/index.html