Index requires coordination to do well. You want one person carrying all the points ideally. When I solo it, I shoot for three turn-ins of 17(+8) for a win in less than 5 minutes. If you're grouped and everyone is doing their own thing you could need up to 75 actual kills instead of just 51 in my case. Granted, they'll (probably) do it faster than the AI bots, but the AI bots being stupid isn't actually that bad a thing. They die, drop a point, then you kill the enemy and pick up 2 points.
Bottom line, I would solo medium risk or I would try to get a pre-made for high risk. PUGing medium just doesn't seem like it would be efficient most of the time. That's using my max hp Inaros and a Tigris Prime for solo, how well you do depends a lot on being able to survive while carrying 15+ points while having 0 energy. Next time I have a boost running I'm going to try a 3 forma pure radiation Arca Plasmor instead of the Tigris. It's so bad when you miss a shot because the enemy dodged at the last second when you only have 2 barrels before reload. I think the Arca will be more effective against medium risk enemies even if it takes 3 shots to kill late in the match.
As for bounties, some stuff you get from them is useful immediately. Gara, for instance. There's some really good mods in there too, even for new players (or especially for new players that haven't had a vet dump a bunch of the must-have ones on them). I'd still focus on doing the star chart first and foremost, but if you need a break bounties are a good diversion.