I don’t plan for every character to have their personal arc advance every short session. Sometimes it’s only main story with perhaps a minor recall/reminder of an earlier point.
I draft the next few plot points for each character by: knowing the What; have a couple of ideas for How; maybe put in a loose relative chronology, but not deciding When.
So I can always throw in another characters What and semi-improvise with their most suitable How. The missed character plot point can happen next time.
Depending on the players and the story: The king wouldn’t demand the Orb if the player was unavailable, or another player would magically have had the Orb the whole time, or the king would get upset and the adventures would now be wanted criminals at risk if beheading for refusing to give up the Orb, or the kingdom would fall and the Orb would implode and warp everyone to a separate dimension made of goo.
All my groups are very good at showing up. Sometimes they can only show up online for an irl-table, or half the session, but we make it work.
If I had one be a no-show a lot I would probably suggest they DM a mini-adventure. It might make them more engaged if they’re the omniscient role and have to do the planning, or they’ll quit, or they’ll learn to appreciate the DMs time. Depending on their reason for being unreliable I would perhaps ask them to pause and return when they can prioritize the group more. And I would definitely make their arc detached from the main story and the other characters.
I’ve only invited people who have enthusiastically agreed to take the time for the campaign, and are proactive in creating their character ahead of time. The players who left so far told us several sessions in advance, so I could weave their personal finale into the story.
Not every group or player is able to do a set schedule, some tables can only book one session at a time. We make that work too.
I second the horror/scary movie trope suggestion. PCs being the hunted instead of the hunters means that the DM can keep the pacing tight and not getting derailed, as the enemy will find them wherever they go until they die or escape (or defeat the monster).
If you place them in a contained area where they have explore to find a way out, they can lay traps to slow down (or defeat) the monster, they can find clues as to the monsters strength or weakness, they can try to hide when they hear the monster drawing neat, they can find slightly helpful items (batteries that can be used for a flashlight or in a baby monitor as an early warning system, slippers for better sneaking but worse running, vodka for temporary HP but worse constitution, scissors or letter openers to distract in close combat for a chance to escape) but not straight up weapons, they can take damage to deal damage, or take damage to escape whenever they get found. The monster will let them think they’ve gotten away but they’ll never be safe, it will always find them while they’re in this location.
The goal can be survive until sunrise, or try to get as many people out of the haunted house as possible, or survive/beat a set amount of the attractions at a demons fair to be released. It’ll be lika a hit+run or cat+mouse game.