We've figured out that we enjoy more playing mini campaigns: you can play them in one gulp, less snowballing, faster hero change.

Does anyone have a good variant in mind to be able to split HoB/LoR/SoN, so one can flush heroes and equipment after the first act? Like it's done in MoB/CtR?

All the questa can be played as standalones. You could hypothetically take the rules from chains (5XP and I forget how much gold) and then when you finish the first quest just keep going. The big issue there is some of the campaigns wull assume that one side or another has a relic, or that you ally will be at a certain power, etc.

To solve this, you could do the following:

Open the campaign book to the first quest:

Roll to determine winner, hand out victory spoils accordingly. If there is a variable reward (e.g., "For each villager the heroes got off the map..." roll for that, too. Do a campaign phase (spend the gold and XP). Winner chooses next quest, repeat until you have arrived at the quest you want to start playing. Congratulations, you've just randomly arrived at a possible starting configuration for that quest.