Sadler and Regan (2016) Chess for Life has some good discussion on this topic.

I used to vary a lot, but I started to get much better results when I played the same classical openings against both weak and strong players. My familiarity with the opening was much more important than any surprise or unbalancing effects. In effect the weak players were helping me prepare for the strong players. It's true the weak players would sometimes earn a draw. Just learn the endgame and you can sometimes win those despite the equalish opening, if not c'est la vie. There's something to be said for playing correct chess all the time. I suppose the calculation is different if all your games go into the database.

There are other approaches, e.g. GM Alexander Ivanov as black vs 1.e4

• 1...e5 vs GMs, in particular the Zaitsev Ruy Lopez, and inviting the Nf3-g5 ...Re8-f8, Ng5-f3 ...Rf8-e8 repetition.
• 1...c5 vs IMs, mostly Classical Sicilian if I recall correctly.
• 1...g6 vs others.

In a multi-round weekender it's a good idea to have a backup opening, in case an opponent might spend a short time "booking up" versus something you played in an earlier round. I remember winning one black game in a swiss with 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Nd4!?. Two rounds later a strong expert played the same white moves against me, clearly he was expecting the same line. I varied with 3...Bc5 4.c3 Bb6!? and he didn't know what to do. (After 3.Bc4 I also play the main lines 3...Nf6 4.Ng5 d5 5.exd5 Na5, 3...Bc5 4.c3 Nf6, and some other things, but then again I have been playing this opening for over 50 years.) This I think is the best way to defeat special preparation. Instead of playing a completely different opening, play a different (sub-)variation within the same opening. You can still rely on your understanding of the positions, and your opponent has to think rather than bashing out their prep.

Also as white if you mostly start with 1.e4 it's good to have one of 1.d4, 1.c4, or 1.Nf3 available. If you mostly start with 1.d4 it's not necessary to play 1.e4, varying with 1.c4 or 1.Nf3 from time to time is probably good enough.

Perhaps, just always bring your best?

  https://www.youtube.com/watch?v=ua_TZ84hmEA