What is a good flexible opening for beginner with white? After I learned the Sicilian, I started to love to play as black, while before it my win rate with white was bigger than black (as expected due to white first move advantage). When I learned Sicilian exists, I started to win a lot as black (also in my low level of play I rarely see Sicilian used against me...)

I don't bother learning all Sicilian variations, I just move the C pawn first, and 'go with the flow' as needed, and with experience learn some of the more unusual moves (like moving pawns or knights in counter-intuitive ways after I notice in analysis why they were good idea, and then I learn later that it is a named opening).

How I do that as white? Right now all I do is spam Wayward queen, that sucks, but seemly other players suck more and lose to it anyway, the problem is that Wayward Queen attack is entirely a gamble, if other player sucks, I win, if other player is good, I lose.

I want some better white opening I can rely on as beginner, like I do with Sicilian for black.

I know there is another question about white openings, but that question talks about lots of openings, I want ONE flexible opening I can rely upon and learn its variants as I play, like it happened to me with Sicilian.
From your question it seems to me that you should focus on learning general opening principles (control the center, develop pieces, don't move the same piece twice, castle...), study tactics and learn how to develop a plan. These will be useful in all kind of situations you encounter during a game and if you follow them you will end up playing an "official" opening naturally.

Studying openings is not useful for beginners because:


your opponents will rarely play the moves you learned
without knowing some general ideas you will basically only end up memorizing moves without understanding the ideas behind them (which is particularly useful if your opponent makes a worse move)
generally, opening knowledge only has a very small effect on the level of play and only becomes relevant at higher playing strengths, let's say from Elo 2000 or so


If you are happy with the Sicilian, by all means continue playing it, but be aware that this is among the most complicated chess openings out there and you are far away from understanding it.

As for what to play with white, I suggest to first figure out what kind of player you are, or what kind of positions (e.g. open/tactical, closed/positional...) you like. For instance what do you like particularly about the Sicilian?