Ender 3 can be a bit finicky. Make sure to check all corners at least twice after leveling. When you level one side, another can be out of level again.

You can also try a different bed material. A glass plate is more flat than the flex one.

If your printer is fully stock, replacing the bed springs either with silicone spacers or stiffer springs can be a good value upgrade. Keeps the bed from unleveling itself while printing, and IMO it also makes the initial adjustment a tiny bit easier. A bed probe is an immense quality of life improvement though, if you can afford to spend a little more money, since it both compensates for less-than-perfect bed leveling and also for any unevenness in the bed surface.

On the subject of upgrades, stay away from the "all-metal extruder" which used to be popular on Amazon etc. The stock Ender 3/v2 extruder is poorly designed and breaks easily. The metal version of it doesn't break, but it's still has the same bad design. If you're going to invest money in a new extruder there are better options.