[RPG] As a DM, how can I manage a group with different amounts of free time

I would recommend doing the following changes.

1. Expect the characters to reach level 12.
2. Give the characters very powerful magic items.
3. Reduce the CR 17 big bad to a CR 15 big bad. If the big bad needs to be level 17 to have access to a spell, ability, or collection of spells, then give the big bad an item which replicates that spell. (For example, perhaps the power comes from the building they occupy, combined with some artefact, rather than their own skill) Alternatively, you can reduce the CR by giving damage vulnerability, or fewer HP.

This should allow them to level up over 2 or 3 sessions, and give enough fights between levels to feel the achievement. The extra magic items will allow the big bad to be a challenge but beatable and you won't have to adjust your planned content too much. My advice is partially based on the fact that the published adventure themselves don't actually take characters to level 20.

Ask your players how they feel about it

It's possible they don't feel any slowing down (or are ok with it). Maybe a big group isn't a problem for them.

If they want to stay as one group, ask them to help you to make the game flow better. That include:

• Knowing the rules. Not necessary all the rules, but enough so that they know what to roll most of the time.
• Knowing their character and their number. No by heart of course, but they should know where to look for that sudden Natation skill check.
• Knowing the rules specific to their character like how to use sneak attacks, what their spells do, ...
• Still being 'active' when it's not their turn, by listening to what's happening and thinking about what to do.

You can also prepare yourself before the session. For instance:

• Have the battleground and the 'flow of battle' ready before the session

• Have all informations quickly accessible. I personally have a paper with a short abstract of every NPC to be in the battle. All the attack and defences values, HP, equipment, special abilities... in the same place, always visible to me.

Since you're on Roll20:

• You can use the macro included in the character sheets (or build your own, or make your player build them) to make the game faster. Things like attacking or rolling for Initiative can as quick as clicking a button and reading the result.

If they also feel they're too many, you can split the party.

You can for instance play the two group in the same campaign, with one group going after one half of a McGuffin, while the other track a group of bad guys wanting to use the other half of the McGuffin to gain power beyond imagination.

An other possibility is to split the party, and run the same campaign twice. It greatly reduce your preparation load (even if they follow very different path, you can reuse assets like map and NPC without the players noticing), but you get other problems like not remembering which team did what in that village, or spoilers from one team to another. Careful note taking and a simple explanation to your players should avoid both those issues.

In both case, you need to see about your schedule. Do you run twice as many games? Will it be one group one week and the other the next one? It's for you to decide as the group.