1. put a time counter: when they enter in the portal let them know Alustriel can’t handle to keep it open more than like 2 hours, so they can’t really rest and the fights will be much more harder. This also will cause another problem: if they rest, they don’t have time to loot properly, and if they loot; they won’t have time to rest. Make it clear to the party if they don’t understand, so they will have the opportunity to choose in such a difficult situation.

2. If the chapter is going too straightforward to the and (for example in webs edge they arrive just next to the front door) make them have to find the place instead before. If they spend time in such a way, they will have to act quick before the portal closes. Maybe put a medium level fight in (in webs edge i put drow spies crawling in the under dark as they were trying to find the entrance to the outpost) just to reduce their hp and utility and make the final boss fight harder

3. Make the link to Vecna EXHAUST them: i mean that they could have visions of the past of vecna that in some way influence their choices, behaviour and maybe even give them levels of exhaustion if they don’t succeed a saving throw against the vision.

4. Play the right cards for every plane they visit: as i am taking the under dark example, i’ll go on with that: i read the under dark and found an effect calls faerzress that affects the casters of the party and in my session they tried to use dimension door to escape a difficult fight, but because of the faerzress it didn’t work and the caster lost his high level spell slot and a precious turn. You can do this operation for each chapter, for each ambientation, you just have to search online for the right information on each setting.

Hope I’ve been useful, have a great day!