It's fun though.

I'm currently on stage 82 and grinding coins at stage 65. Based on my experience, I get about 170,000 coins per clear at stage 65. It auto-saves, but it's a good habit to manually save by pressing the gear icon before exiting to prevent occasional save failures. Blue gems are used to increase hero stats in the hero tab, or for transcendence and awakening. Pink gems are used in the shop, but that's optional. Green gems are used for pulling buffs in the ability tab. If you enter the dungeon at the bottom right and successfully defeat the boss, you'll get relics and relic chest keys, which you can use to equip each hero with their gear. For miscellaneous items, you can upgrade the corresponding gear in the relic tab. As for the Lucky Bus, you'll figure it out when you see it. It resets weekly, so free-to-play users don't need to rush if they're not progressing. There are some ads, but it seems fun.