Hi there
One of the best pieces I have seen on this are from Rand's The Massive Ranking Factor Too Many SEOs are Ignoring Whiteboard Friday. It dives into a few indirect tactics and also helps walk your mind through how you can showcase your efforts.
The best advice I ever got was to experiment with ideas or hypothesis on what you think may move the needle - whether on your site or starting a new one just for the sake of experimenting. The beauty about SEO and digital marketing right now is that no one has it "nailed down". There are so many opportunities that there is no right way of doing anything yet.
I will say that you should always mind your annotations in Google Analytics - that way when you make a change you can see what changes had the most impact over time.
You asked...
"They speak of 'indirect but measurable', but does this signal continues to increase or will it remain as it is. I am anxious whether user engagement as ranking factor is further decisive or not?"
No one knows yet, I can only imagine (again, assuming) that usability and user experience will increase as a ranking factor over time as so many KPIs and quality signals can run through those two items, and there are so many different angles you can look at it.
The best thing you can do right now is provide a stellar user experience, engage your audience, empower your team to be thought leaders, and embrace change as it happens in the digital marketing world.
Hope this helps a bit! Good luck!