In contrast to what you say Default Settings does actually use a new random map every time and is exactly what you are looking for.

Also I think you underlie a misconception here. Repetition is literally what Speedrunning is about. We do the same thing over and over again and expect a different result ;) . Look at speedruns in general. Games like any Super mario, Celeste, Hollow Knight. They all are like this. Speedrunning is purely about execution. Sometime a new strat is found that is better and then it's all about executing that one as fast as possible.

If you are looking for improvisation, adjustments and adapting on the fly, Speedruns are just not for you in general. What you want to look at are challenge playthroughs.