Dear dungeon masters, what would you do in this situation?

My players ,level 15 with boons and tons of magic items, are currently in battle with an archlich. Esentially a buffed version of Vecna.

We have been in this combat for the past 2 sessions and are about to head into the 10th round. (we play 3ish hour sessions and are kinda slow)
My players have been suffering through the lich's entire spell list. Through having 3 reactions to cast counterspell, using 2 legendary actions to cast a 4th level or lower spell and the sheer amount of turns that have passed I have been able to throw almost everything I have at them. The lich had a full level 20 spell slot list and only has 2 1st, a 5th, a 6th, a 7th and an 8th level slot left. Aside from that I have used a few spell scrolls, including a 6th level chain lightning and a 9th level power word kill ,which actually killed one of my player's characters. And finally have been able to burn through all 5 of his legendary resistances

My players know alot about this enemy now. Either by them figuring it out or by me telling them. His movement, AC, many of his saving throw modifiers, general max HP, spell save DC, his unique abilities and what they do and what their DC's are, the status of his legendary resistances and the fact that he has 5 legendary actions and 3 reaction every round.

They are struggling the most with a feature that I lifed straight from the vecna stat block. The ability to heal 80 hit points when using a bonus action to teleport near one or more creatures. Not just because it heals him. But I added a feature that, when he hits certain HP levels (150, 100, 60, 30), it causes him to create a shield around himself that makes him immune to all damage except radiant. Using radiant breaks this shield and allows regular damage to go through again.
The reason they are struggling with it, is because the last three turns they have managed to get him to below 30 hit points, before it became his turn again and he could heal back up to around 100-110. This largely came to pass because of bad die rolls. With only a cleric and divine soul sorcerer in the party of five, there are only a few characters that can deal radiant damage to break the shield (barring the use of spell scrolls, which I have confirmed they still have in their posession). They have also summoned an 8th level summon celestial that is helping by dealing tons of radiant damage (if it isn't suffering from bad die rolls such as missing all 4 attacks during the final turn of round 9).

At this point, both the lich and the players are at decent hit points, but otherwise pretty low on resources such as spell slots. But when the 9th round ended without them being able to slay the lich, and he was able to heal again during his next turn. This caused one of my players to become quite pissed off and pretty much ragequit the session, which we had to end early. They were talking about maybe trying to flee the battle, others wanted to keep it going when we pick up next session.

I talked to the players and apologized for perhaps overdesigning this lich. But I was just trying to give them a true challenge. Three out of my 4 players that attended the session understood the idea and think its still doable while the one who got angry at the end of the session simply called it bad design.

I am now kinda stuck with what to do. The party IS balancing between a hard won victory or a possible TPK. Really coming down to the die rolls at this point. Personally I think they can still beat it. They almost beat him the last three turns if it wasn't for lucky rolls on the lich's part or particularly bad rolls on the player's part, like that celestial missing all 4 attacks.
I usually like to let things go as I had first planned it, but I am wondering if some of you would perhaps still change the stat block of the lich at this point in the battle. I could change how the healing works or how the HP Threshold shield works. But they have pretty much figured out how those features work and if I change it now, they will know im going easy on them.

Should I consider changing the stats to save my party or should I let the dice gods take it from here regardless of if this makes that one player ragequit?

A lich's top priority at all times is survival -- if the combat has ground on this long and could still swing either way, I don't think it would be unreasonable for the lich to try and bail. They generally prefer better than 50/50 odds

Note that they don't need to succeed on their escape attempt, if the teleport gets counterspelled or whatever. The simple act of trying to flee might be enough to rally the spirits of the PCs and make it feel more like they're just one good round away from winning, rather than one round from losing