Other than damage up, you'll also like having more damage up and even more damage up. When you have that, you should try to get a lot more of ... OK, OK, you don't actually need three mains and nine subs of damage up. For the Splatterscope(regular or Kelp), I tend to use one main and six subs of damage up. That is a good place to start. Beyond that, it's a matter of preference, but here are some abilities worth considering and why:
Damage Up. Yes, more of it! I feel like the one main and six subs I already mentioned does a fine job already, so I don't usually go for more, but having another main or three subs can make a difference with slightly faster OHKO shots as well as counter people running around with a bit of Defence Up.
Cold-blooded. Because nobody likes the government spying on them. Really, being tracked makes it a lot easier for the enemies to keep you from doing your job.
Ink Recovery Up. Chargers use up ink quickly, especially if you want to use your sub weapon. I like to have one main or three subs of IRU to smooth help with this.
Ink Resistance. This just makes a lot of situations more bearable for any weapon that doesn't have burst bombs.
Tenacity. Unless a game is going really poorly, chargers tend to stay alive for longer while teammates die, which makes this ability give you a lot of special charge.
Bomb Range Up. You already know this is nice. I wouldn't stack a lot of it, but one main ability is nice to have, because it's exactly what's required to make the difference in a lot of situations. I think the stages were even built with this in mind.
Ink Saver (Main). Helps with the same sort of issue as IRU, but with less versatility. I wouldn't recommend this over IRU, I'm just saying it isn't bad.
Swim Speed Up. As a charger, you won't be moving as much as most other weapons, so this isn't something I would highly recommend either, but SSU is also never bad to have. If you personally prefer to have extra speed while moving between your positions on the map, go for it.
As for your issue with getting flanked, there are three things I can suggest.
1. Stop the flankers by hitting the shots before they get to you.
2. Practice at glancing at the map, so you won't be surprised by flankers.
3. If you know someone is about to reach your spot and you won't have the advantage, don't be ashamed of getting out of there, either by retreating or even super jumping to spawn. It is so fun to make people waste Krakens like that.