[RPG] Need way to physically track initiative in face-to-face D&D game

This is not a problem for most effects. Most of them last “until the end of your next turn”, which for effects that benefit something you want to do on your next turn is plenty long enough.

The only case where that produces counterintuitive results is when an effect disadvantages your opponent, as in your defensive spell example or a monk's Stunning Strike, because if your next turn comes before they next act then the effect is “wasted”.

This second class of effects is easy enough to identify on-the-fly though. Since you're already using a variant rule, it's easy (and totally true to the spirit of the rules as written) to house rule that such effects last “until the end the next round”, giving a full round plus a few initiative counts of effect, fully covering the opponent's next action.

The only drawback of that is that it might cover more than one of your opponent's next turns, if the initiative counts fall that way. Why do I say that's a drawback? Because the enemy can do the same to you, if they use such an effect against you and the dice go their way. However, that makes it pretty fair: the PCs might occasionally get the double-dip benefit, but so can the enemy. And the dice are likely to more often go in the heroes' favour, since that's the overall design of the rules.

I use the following house rule:

All monsters act on the same initiative. Player initiative determines who goes before the monsters on the first round, but after the monsters act the players can generally go in any order (usually in seating order, clock-wise around the table) because they will all go before the monsters' next turn anyway.

Notes:

• Since monsters of the same type normally go on the same init anyway, this just takes it one step further.
• For the monsters' group initiative: I use a 'passive check' (10 + average dex bonus) for extra-balance. (if desired, average dex bonus can be proportionally derived so that a high-CR monster isn't unduly altered by a single low-CR minion).
• You can instead roll the initiative, but it can be less balanced (i.e. easier or harder) when all the monsters share the same roll. Also, a passive check allows for speedier starts, and the players' rolls matter more.
• In practice I have the players roll init and say "Raise your hand if you beat init 12" (for example), then allow those that raise their hand to go initially (usually in seating order).
• Yes, this could allow a lucky player to go twice before an unlucky one went once, but they're competing against the monsters (not each other). Plus, this method actually doubled the rate of play, so the end result was more playing for everyone.
• The reason it doubled the rate of play is because it also allowed me to easily skip past anyone that wasn't ready (coming back to them once they were ready).
• The phrase "You'll all get to go before the monsters' next turn anyway, so I'm not too worried about the order" seems to garner buy-in even from traditional players.
• I've used this house rule for over a decade. It has done well through many discussions on the old WotC boards, dozens of conventions, and dozens of campaigns with different groups.