The HALion Sonic SE 2 - like all sounds bundled with notation software - necessarily contains compromises. Its oboe, for example, is too loud to most people’s ears. The upper strings (violins and viola) have much less convincing tones than do the cello and c.b. - as you will probably have found.
Remember, too, that the c.b. and cello are inherently quieter instruments - indeed the c.b. is the quietest instrument in the symphonic orchestra.
The default settings in the mixer should work reasonably well. I tend to start with each instrument marked mp at the start of its staff, and the Mixer’s faders in their default positions.
You might want to experiment with a ‘test’/template score containing just the strings - originally set up that way, and actively experiment to discover how much you need to tweak the faders in order to get the balance you want.
Although, generally speaking, spending a lot of time in the Mixer shouldn’t be necessary, its setting are persistent and you may only need to change the balance to suit once per composition.