SteveC
Doing the best imitation of myself
This sounds like a really rough situation and I feel for you tremendously. I play exclusively online, but I have a document with what my character can do with the edge cases included and it helps me tremendously. But I feel for you because we have one player who is a really smart guy in real life but in D&D ... he's just challenged. I sort of got that feeling from your post. The rest of the players in my group play complex characters and get the rules, so we can help him out. If the whole group was like that, I don't know what I'd do. There's all sorts of "player management" techniques you can use, from having players have reference sheets to giving them a timer for turns to just having them tell you what they want to do and have you manage the character but I don't know that any of that would really help.Tedious as opposed to being fun and exciting. The player characters in my current campaign have reached level five, and most combat encounters last 4-5 rounds. Keeping in mind all my players except one is using D&D Beyond off their phones instead of having a character sheet, here's how it goes. Player takes an action, often has to hunt through their character sheet for special abilities, modifiers, or spell effects, player takes a bonus action, and then tack on the effects of weapon mastery it takes a while to get through combat even if it ends after 4-5 rounds.
From what you say here, I don't know if there's a rules solution, though. I think that there's a part to D&D where it requires the players to do some of the lifting to make the game run, and that includes some level of rules mastery. I don't know that there's a 5E solution in it for you. From my perspective, it seems like changing back to something like BECMI or moving two an entirely different system such as one of the Blades in the Dark or PbtA ones might make it work, since those games play very much with a conversation. In our group, we played Feng Shui recently, and the player who couldn't get the rules to work just clicked and had a blast. Perhaps in his case the rules got in the way of an excellent imagination. Maybe find a game like that? The game Grimwild might be to your group's liking, since it's much more narrative, but still has the chassis of D&D classes.In the interest of fairness, some of these problems are exacerbated by my players. Of the six players, only three of them have a firm grasp of the rules and their character's ability. I think two of them don't even own a copy of the PHB. One of them had a completely different idea of what a Druid was all about in D&D, seemingly wanting what was essentially a Hunter from World of Warcraft. I can't blame that on the rules.
I guess I'm trying to say that I really sympathize with you but I don't know that this is a 5E design issue.