I've seen enough examples go past to think that single subject rules tend to be honoured in the letter but not the spirit, but equally if a frankenbill can pass

I've seen enough examples go past to think that single subject rules tend to be honoured in the letter but not the spirit, but equally if a frankenbill can pass without a single legislator invoking that rule, then it's entirely possible its components were all sufficiently popular the outcome was fine. So I think it's entirely possible that you could come up with a single subject rule that was enough of a net win I'd be in favour, but I suspect you'd want to look at how the existing ones have been gamed and work out which those cases want to have an official way to do them and which you want to reword things to make harder.