Never let go of the throttle. This goes for all tracks, every speed class.
Start wide and airbrake + full steering earlier than you think, aim to slide through the corner as opposed to simply going around it.
As soon as you reach/cross the apex of a corner, sideshift in the direction you are turning, this will greatly improve your laptimes/speed around the track for various reasons.

Zone mode is a little different. When the speed gets high you need a wide entry to the corner, but you also need enough of an angle to make the turn, so a lot of the time it will involve
pointing to the outside of the track and turning earlier such that when you reach the wide point of entry, your craft is already part way through making the actual 'turn' that it needs to.
You'll get what I mean if you try it I think. Middle-ish of the track, point it slightly to the outside of the track (away from the apex) then turn in early. You make corners much more easily that way.

There are many many barrel roll opportunities on sebenco. There's a minimum of around 9BRs per lap without turbo, depending on speed class, but for the time being concentrate on the barrel rolls you know, and focus on your lines around the track.

Is there specific speed class that is giving you trouble? Techniques for each track vary wildly with different speeds, so more in depth advice may be given if we know which one you're having trouble with.

More than anything, make sideshifting a habit. Sideshift every single corner in the game till you can't help it. It's more or less the most important part of the game as far as lap times are concerned.