I was interested in running this game but my vision was to use user decks but from a new save file so I’d have to build my deck throughout the run. It doesn’t look like that’s a thing though, has anyone ever tried it? Would people be interested in trying it?

There was somebody, who had that idea a couple of years ago, but it has never been attempted in the end. The game has been very inactive for years, so there isn't really any demand to justify adding something new. Categories aren't just getting added based on new ideas in general. There have been other ideas for new categories in the past. If all those got added we would have an abundance of categories, which all would have been run by probably just the person suggesting it and nobody else. So unless there is a sudden resurge of players, that would all be interested in running the game that way and also doing so, don't expect there to be a leaderboard for it any time soon. But that shouldn't stop you from running the game that way, if you have interest in doing so. A game doesn't need an official leaderboard in order to be speedrun in some other way. I have done speedruns, that had no official leaderboard, myself like DM Battle Pack Draft.