Nerfing a Lich encounter for 7th level PCs

[1Pirate]

So, group of 5 PCs: Dragonborn Paladin(Ancients, sword and board), V. Human Rogue(Arcane Trickster), High Elf wizard(Abjuration), Half-elf Shadow Sorcerer, and V. Human Cleric(Life). All are capable of inflicting damage that doesn't run afoul of the Lich's immunities and resistances(although the Rogue might have to get into melee if he shoots more than 3 crossbow bolts) and they are currently immune to fear. They won't exactly be fresh before the encounter, but they have a pretty good idea that they should be saving their high level spells and other long rest resources. The cleric is familiar with Lich capabilities, but they won't know they're facing a Lich until the actual encounter.

Situation: The Lich is attempting to assemble a MacGuffin and has broken into a temple(so no lair actions) that contains two pieces of said MacGuffin. He has gotten one and is attempting to get the second when the PCs encounter him. He will have just fought the temple's senior clergy and won't be at his full strength.

My thinking is the Lich will fight for 3-4 rounds. If he hasn't bested the PCs by then, he'll cut his losses and use a teleport scroll to leave with the one piece. If the PCs beat him, they regain the one piece, and set back the Lich's plans(his phylactery is far away and the PCs are in no position to find it at this point).

I want the fight to be difficult, but winnable if the PCs play smart and have a couple rolls go their way. So, I'm thinking the Lich will be tapped out on his high level spells and maybe have lost some of his legendary resistances, and down some hit points. My question is how much should I weaken him? Should I just clear out his 5-9th level slots or do you think a Lich could wipe them out with just his cantrips and legendary actions? How many hit points should I leave him at? Should I scrap this idea entirely?(in terms of the narrative, the PCs had been hearing about this guy and his plans, but no one seemed to know he was a Lich)

[Unoriginal]

Narratively, is there a reason for the Lich to do this themselves rather than send minions?

Personally, I'd rather not have the BBEG flee in front of 7th level PCs. Might give them the wrong idea.

If the Lich had to defeat the guardians of the temple before, sure, it can be justified, but in that case they should be leaving ASAP, without giving the PCs time to do anything.

You could say the Lich needed Gate to break into the temple, though, to make them use their 9th level spell slot.

In any case, to weaken a Lich that much, the temple's defences have to be impressive. It'd be nice to have them explained to the PCs a few sessions beforehand, and then the BBEG smash through them in a "oh ****" moment.

[Kadesh]

I'm leading up to an encounter for my parties (4 level 11's), where they have to slay the Simulacra of the Lich who is making a ritual over several points of the map, miles apart from one another.

Because they are Simulacra, I am trying to make it not a top level fight, but still leave it dangerous for them all: I've removed the Legendary Resistance, 8th and 9th level slots, and instead gave them all Great Scythes (Halberds) which can deliver the Paralyzing Touch, and a modified Spell List to remove the Save Or Dies.

The first one was difficult enough: 4 Toll The Bells per turn is a rather horrific amount of damage: 16d12 hurts.

[1Pirate]

Narratively, is there a reason for the Lich to do this themselves rather than send minions?

Personally, I'd rather not have the BBEG flee in front of 7th level PCs. Might give them the wrong idea.

If the Lich had to defeat the guardians of the temple before, sure, it can be justified, but in that case they should be leaving ASAP, without giving the PCs time to do anything.

You could say the Lich needed Gate to break into the temple, though, to make them use their 9th level spell slot.

In any case, to weaken a Lich that much, the temple's defences have to be impressive. It'd be nice to have them explained to the PCs a few sessions beforehand, and then the BBEG smash through them in a "oh ****" moment.

Narratively, the Lich felt none of his minions were powerful enough to disable the enchantments or deal with the clergy on their own. I should perhaps clarify that this isn't some long-abandoned temple in the wilderness, it's more akin to the Vatican in the sense that it's large, active, and houses a number of high-ranking(read: high level) clerics, so this isn't something he can leave to any old death knight.

The Lich isn't immediately leaving because there is a second piece he wants to get, but if he starts to think the PCs might overpower him, he'll go.