You could buy ' Big expansion + mini expansion + 2x H&M pack (+ Lieutenants for each campaign if you want the figures)' and repeat that every time you finish a campaign.
For example, first buy:
Base game + Lair of the Wyrm + Crusade of the Forgotten & Guardians of Deephall (+ Splig, Zachareth, Merick, Valyndra, ...)
Then once you've finished Heirs of Blood (base game campaign) buy:
Labyrinth of Ruin + The Trollfens + Visions of Dawn & Treaty of Champions (+ Ariad, Serena, Raythen, Bol'Goreth, ...)
Once you've finished Labyrinth of Ruin buy:
Shadow of Nerekhall + Manor of Ravens + Oath of the Outcast & Crown of Destiny (+ Trystane, Rylan, Skarn, ...)
...
The H&M packs I suggested are purely random, the mini expansions I feel are thematically the optimal matches (in my opinion of course) with the big expansion I placed them with.
That way you have some rumor quests, and some new heroes, classes and monsters to add to your campaign every time, while keeping the monsters of the big campaigns fresh for their respective campaign.
I wouldn't recommend buying the base game + Shadow of Nerekhall for example to start out with. For starters, you won't be able to use much of what comes with it during your first campaign anyway (tiles, Citizen cards, the campaign itself, ...) but you also kind of 'spoil' the monsters like Ironbounds which are linked thematically to the Nerekhall campaign by already using them in Heirs of Blood. And as stated before, just start with the base game first, and see if you actually like it
Edited by Atom4geVampire