So, I'll try and update as I (hopefully) figure it out.
You can start by installing soundfonts in musescore. This will give you a variety of sounds and timbres. However, since Musescore is no DAW you have very little to no control over them.
FTM I like these the most:
To add these to musescore:
Anyway, this doesn't give you a realistic sounding orchestra. So, I'll definitely need to look into a DAW. But which one? There's so many of them and since I don't know what I'm doing I don't know what features I need.
Hopefully more answers soon...
So, what a surprise. Turns out I'm one of the fussy guys.
I started using the ensemble sample sets of VPO, but I'm not satisfied at all. They make the music feel as if it's dragging. So, since I don't know how to use a DAW yet, I went with a poor man's solution:
I duplicated all of the string staves in musescore, e.g. now I have two identical viola staves, then I assigned a different solo sample set to each. E.g. the default fluid "viola" to the first staff and then the VPO "solo sustained viola" sample set to the second one. And did this for all string instruments.
Then, instead of exporting the file as midi, I exported all parts as wav files. I.e. now I have 2 sound files for each instrument: one using the fluid sample set and another using the VPO solo sample set.
I have then loaded all of those tracks separately into a single garage band project. Garage band is fine for now, it introduces me to some of the concepts and as all of the Apple toys it's really intuitive and doesn't overwhelm me with 100's of things I have no frigging clue about.
So, now I'm tackling the volume changes and reverb track by track in Garage band. It definitely starts to sound better, but I'm nowhere near anything realistic yet.
PRO TIP: when exporting from muse score, make sure you turn off all dynamics first. You can do this by selecting all dynamics and setting the velocity to e.g. 64 See https://musescore.org/en/node/45851
Then, I use the score as a guide to know where I need to do the most basic dynamics in garageband first. The single individual dynamics I'll tackle later on.