I think uncapped Fischer time is the best setting for live games overall, but I want to share an idea about capped Fischer anyway.
Let’s allow the cap to be lower than the main time, so players don’t gain time in the beginning, but only once they are below the cap.
A game with 20 minutes main time and five periods of 30 seconds byo-yomi would be comparable to a game with 22.5 minutes main time and a 30 seconds increment capped at 2.5 minutes. After the main 20 minutes are over, a player has to play a move within 30 seconds on average, but can use 2 extra minutes before timing out.
Why? To prevent players banking so much time at the end of a game with a series of obvious moves that it takes away almost the whole time pressure.