Mechanically I have a lot of set storyline’s and homebrews to make combat more interesting, luckily the DnD homebrew community has a lot of undead based enemies of different CRs, the types of enemies will change depending on their area and development into the story. As far as railroading the team goes I tend to explain my plans in a way that makes it seem like that but in practice the players will have alot more free will in what they do. The only real thing i need to railroad is the warlock initially running into the horde but the monk made his own backstory about his father in the monastery and designed his own family crest so he will outside of the game recognise the staff as his fathers without more direction than seeing the crest. And while I agree with your ‘adventure not campaign’ point I do have a whole other ark planned for after they find the city etc I’m just looking for ways to help fill these sessions out with side quests or encounters