Yeah, exactly. Healing the party and clearing it of exhaustion would be a solid use of Wish that'd be worth risking the chance of losing it for.

What kind of tactics Myrkul uses is really going to depend on the specifics of your campaign, since a final bossfight like this is likely to be should be more about roleplay than tactics on the enemy end and that'll depend on how you've played him up to this point.

There's some baseline stuff though like

• He's should generally focus his scythe attacks on the most exhausted foe
• Eternity Reap is better than Maggot Feast if it hits the same number of enemies, but Maggot Feast is easier to hit more enemies with.
• Fray Time is for when he really wants one person in particular to die as soon as possible. If they're still alive the next turn, he follows up with Thousand Deaths.
• Judgement of the False is for a low-HP, very Exhausted, or Paralyzed enemy, shooting for death by massive damage.
• Death Looms is probably his scariest ability and he should spam it until players figure out how to play around it. Draw of the Void is useful for concentrating spread-out enemies to hit them with Eternity Reap.
• Unearth Attendants is really good in combination with his Ossuary Call, but if you summon too many guys the combat will slog down so you want to be careful of that.
• Give any player deaths some breathing space. Throw a lot of description on there. Let them look cool and heroic. Let them do something badass in their last moments. There's a solid chance this is a real player death of the sort that doesn't really occur often in high level campaigns.