Been thinking on this since last night.
Seems like we have table pacing/management problems stemming from a large group and a few people who are bogging the game down with procedure.
What I’d be looking to do if I were in your shoes is 1.) reduce system admin time (that is time spent using the system versus time spent playing the game), 2.) simplify my task resolution systems, and 3.) reduce the amount of time between one player’s turn and their next turn. With 7 people, I’d like to hit any given player once every 7-10 minutes and no later than that.
For 1, Reduce System Admin Time, I’d have spell lists on a lined sheet of paper with the spell name, level, spell slots, and a page citation for fast lookup. I’d probably also put post-its in my PH as a bookmark to some of the signature spells. On your turn, if you’re casting a spell, the book had better be open to the spell you’re casting (or else you get a cantrip instead).
I’d also remove the attack roll entirely - skip straight to a damage roll. Since your players seem to have trouble with adding this sort of thing together, I’d also trade my static bonuses for dice; my long sword attack would deal 1d8 + 1d6 (reflecting my strength bonus of +3). Anyone can roll a pile of dice and add the numbers shown. (If you need more monster HP, that’s an easy adjustment).
Finally, as DM, I’m making sure the next player starts thinking on the current player’s turn. “Jim, you’re next after Pam, get ready. Pam, what do you do?”
For 2, Simplify Task Resolution, I’m rethinking all ability checks and Saving Throws. Probably getting rid of Skill Checks altogether and using the Saving Throw proficiency to indicate what abilities the adventurer is good at. Fighters get Strength and Constitution so instead of tracking 25 different skills, they’re just getting proficiency bonus to all Strength and Constitution checks and Saving Throws. You could do this as a static number or place a die on the character sheet. When I DM for kids, I put the dice on the character sheet. This might sound like some crazy adjustments but we’re trying to reduce the cognitive load on the player by doing everything the same - pick up and roll a pile of dice and then add them up.
For 3, Reduce Time Between Turns, this is largely on the DM to push the pacing. But, if everyone is skipping an attack roll, have their spells ready, and don’t have to cross-reference an ability score and a proficiency bonus to a skill, we’ve done half the work already. Just need to make sure turn order is very clear and have the Next Player thinking on the Current Player’s turn.
I’ve run 7 players, it’s hard. I’ve run newbies and kids. It’s hard. I’ve done combos of skill levels. I’ve done attack rolls only (and static damage) and damage rolls only (with monster AC as a damage threshold). I’ve done Cantrips Only with spell levels adding damage or targets. Basically, whatever gets to the simplest system for the group. My preference is d20 + some other die. And practice faster pacing.