I actually run a game similar to yours. Mine is through a local library, but it also has a whole bunch of players (9, at most), and usually I only get a handful (6, maybe 7).
I have a few suggestions, which can be implemented in any combination. This might be a little focused on avoiding the root problem rather than the question you’re asking, but I’ve seen all of these work well to solve similar problems.
Talk to the players
This is a good idea anytime there’s a behavior issue. You can always say “hey guys, I think when we run as a whole group, it’s hard for people to share the focus” and ask them for suggestions. I’ve done this before, and my players generally say they feel better knowing their GM is a human, with human problems. If they have a good suggestion, I would try it for a session.
My groups usually play one of two ways for this issue. In three groups, an absent player means the character stays behind the last place we rested. The absent players still get XP, but the rest don’t have to worry about dealing with more than one character.
The other group I’m in, as a player, is big (10 players). We treat absent players or new players a bit differently: time travel shenanigans are a thing so whoever shows up, their characters are there too. If you can’t make it, your character just isn’t existing right now or never did in the first place.
Either way can work, depending on the group. Again, talk to the players.
Side note: I prep as if I’ll get about 2/3 of the players that usually show up. The GM in the group I mentioned plans for about 7 players showing up on average, then changes that if necessary. Plan for fewer players then the average, but have adjustments for more players showing up (harder problems, stronger monsters) just in case.
Split the group
Originally by @linksassin in chat: link, scroll up a bit and then down for the rest of the discussion.
If you still can’t run a big group, you might try splitting the group. This doesn’t mean split the party, but it might (for me, when I’ve had to do this unexpectedly) mean running two or three sessions in the same world that have different subsets of players. Again, talk to your group. Please. Everything is easier if you talk to them, and listen to their responses.
Get a co-GM
Another person to help, whether they just help adjudicate actions for other players or they run one group through a scenario, can be a lifesaver here. In my big group, I occasionally get the GM for another campaign with a similar group (he plays in my campaign, I play in his) to help me look up a ruling while I deal with someone else. It saves time and keeps focus on playing.
Use a chat
You say you’re playing online. For my game, now that we’ve moved to video calls, I use the chat feature to adjucate multiple actions. The first person to say something over voice chat gets the focus, and I deal with that person before I move to another person. Anyone else who isn’t in on the current interaction who wants to add something or do something puts it in the chat and then I go through the chat in order and work with those players to resolve actions. We also usually put dice rolls in the chat so as soon as I tell someone to roll I can move on.
This doesn’t work quite as well in combat, but we use something similar for that.
Remember you’ll do fine
You will be fine, whatever happens. Players probably will accept “I need to figure some stuff out about our game, let’s try this and see what happens” or “here’s three things this player told me they had trouble with, how would y’all fix them?” I’ve done this, other people have done this, there’s plenty of ways to make it work.
Whatever happens, work with your players, be responsive, and everything will end up better than before.