Make game harder and harder. What is the ideal way to do it?

Hello there,

Im currently working on a new game. The game has no lvls in it and its a where the goal is to break your highscore. I have been thinking of a way to make it harder and harder over time. Here are my ideas how I should make it:

1. make a timer after X seconds spawn actor every 1 seconds and so on. This way is working but after a while you realize thats is going to be much "after X seconds". Its also more like waves and not a smooth change.
2.Making an attribute called Minus.
Rule every 1 sec change attribute minus to -0.1 (starts with 12 as an example)
Then another rule.
every random(1,minus) spawn actor (this is also working but it has some bad things. You can't as an example spawn multiple actors and it has an limit how hard it can be)
3. Rule when attribute score is over X spawn actor every sec and so on(this is not ideal because then you can wait play how long you want without its getting harder and harder)

Yeah thats the ideas I have. If you wondering something fell free to comment.
If you have another way to do this please say it.

Thanks,
Fredrik