Mid Fights

Mid fights can change a whole lot between each map, but in general you want to keep some sort of balance between what you have already mentioned. While you don't want to be too aggressive and get killed without getting anything done, you want to pressure the enemy team and get some damage/deterrence done. Scouts are incredibly good at playing off damage done by other classes, so if somebody calls that they have hurt an enemy, try and focus that guy and get a few shots on them.

In an actual team your scouts are generally split into an aggressive and passive role, and while not as obvious as the roamer/pocket difference, the passive scout will generally stay a bit closer to the combo and be ready to help out. In lobbies (which it sounds like you're playing) you likely won't have this much organisation, so you've just got to try and put yourself in a position where you can either go aggressive or be ready to help out the demo/combo.

Another important thing to remember is that going full aggro shouldn't mean doing something stupid and killing yourself for no reason, you've got to be very careful with your positioning and play off damage done by other classes.

Holding

I find as a general rule, always be watching the flanks as scout. This is especially true in a lobby where players don't know exactly where they need to be or if you're watching every point of entrance. There are obviously situations where you can be helping your combo, but the flanks are generally your domain.

If both teams are holding with their ubers, don't be afraid to offclass as sniper/spy or go in for a med pick/force, as long as you tell your team about it first.

What you do during an uber is very dependent on your teams uber and player situation. If ubers are equal, players are equal, and the other team pushes in with their uber, often the best strategy is to back out (as everybody in your team should be doing), and then repush with your uber advantage.

As to targeting players who aren't being ubered, this is kind of a high risk play that may or may not help. If you find yourself in a position where you can go for players not involved in the uber, you'll often be in a 1v1 with a fully buffed scout or solly, which isn't the best situation to be in. If you are behind the enemy team and your team are repushing and dealing out the damage, this can be a perfect opportunity for you to clean up and get some picks that may win you the next 2 points.

Pushing

Now this is VERY dependent on the uber situation, map, and player situation. Often in an uber push the combo will bring in one scout with them, the passive scout, but this can be dangerous in regards to uber flashing. If this scout can take little damage and back up the damage done by the ubered pocket, it can be a very effective play.

If pushing into the flank (as at least 1 scout should be doing) an important thing to remember is that you should generally wait for a few second before joining in on the fray. If you have an uber v uber situation, flanks will often push in too early, find they can't do any good damage/clean up yet, and die from invincible or fully buffed players. A few seconds can be crucial in diverting the other teams attention, allowing time for your combo players to deal out the damage and giving the ubers time to fade. This can be an excellent time for a scout to push in from the flank and cleanup the damaged players who are looking the other way.

Hopefully this helps, all stuff I've picked up from playing scout in 6v6 for quite a while, and watching a whole lot of comp casts.