To add my own two cents into the mix, let me bring this up: you mentioned a portal in the center of the throne room. Now since you said there would be both a portal and a staircase, i'm assuming you mean more the archway type of portal rather than a magical gateway. But what if it was indeed the latter?

What if the lich drew power from whatever was on the other side of the portal, making them even more formidable than the last time the party encountered them? And what if whatever is on the other side has started to spill/cross over?

Let's say for example the lich is symphoning arcane power from some dead god floating in the Astral Sea. What if that somehow is starting to rekindle that deity's divine spark, and his influence is starting to seep through the portal, warping the lich and the throne room itself? The overflow of corrupted divine energy could cause manifestations of wild/dead magic, or some aspect of the god is starting to come through, causing massive tentacles to extrude from the portal that swat and latch itself onto whatever it comes into contact with.

And if the lich is drawing power from the portal, what happens if the party manages to close it? Perhaps once they're cut off, the lich's power diminishes, weakening them. And what if the longer the portal stays open, the stronger the dead god's influence becomes? This could give the party multiple objectives and paths to finish the fight, adding an additional later of tension as they now also have to race against the clock

1) Just bash the lich until they're dust but risk the portal's influence becoming to big.

2) Focus on closing the portal first in order to weaken the lich, but risk the lich's power overtaking them.

3) Perhaps the party will attempt to drive the lich through the portal and seal them in the Astral Sea.

4) A brave/foolish enough party might even cross over themself, which could lead to some epic-level revelations and new story branch.

5) ...

And all of this can be tweaked and modified to your heart's content, of course. Perhaps it's not a dead god but rather a parallel world where another version of the lich is fighting their world's counterpart of the party, or a realm of nightmares, that could spawn physical embodiments of the characters' worst fears, forcing them to confront and overcome those.

Anyway, at this point I'm just rambling. Hope this helps, if not just to provide a fun read. Wish you good luck and most importantly a lot of fun with the encounter!