Thanks for your words about the game!
As for gold... First, read the book of fusions in the library. It will help you understand which items really need to be leveled up.
To test yourself, you can go through all the locations where battles are possible and see if there are any rare chests there that you have not yet found. Rare unfound chests are displayed at the bottom of the game screen, on the right. Usually this is a chest with a number. If there are only 3-4 chests with an infinity symbol, you have already found everything in this location.
Regarding difficulty levels: you can adjust the difficulty level yourself using artifacts. Because there are artifacts that simplify the gameplay and vice versa. You can enable or disable skills that you have already discovered. Same with clothes.
Regarding the shortcomings you listed: try to study the game world more carefully.
The library has good books with tips and explanations.