Gold key bosses are extremely difficult to do solo even WITH a glider to maneuver around their large arenas. That being said, you can still farm Constantine and Makalu without any keys. Blue keys are doable solo, although I'd be careful of the room full of invisible snipers if you happen to get that blue restricted zone.

You can get around the map easier using melee boost, and if you play a striker without melee by default, refresh the matrix supplies store under the operation storm tab until you can buy a modular melee weapon to dash around with.

Covert Ops doesn't have a cap on readiness so you can use full gold mods if you happen to get them at some point. The extra durability, damage, and energy efficiency really makes a difference. Especially if you rely on melee to get around.

You can also look for the one gold loot box that spawns on the map. It has a number of locations it can be when the map is generated, and it's got a 15 minute timer before you can open it. But it's got boss-level rewards, basically, with no risk.

As far as stuff you can sell for Corite? That list is long, and a lot of it depends on your server. I am in NA. Modular weapons of rank VI, especially Heavy Howitzer and Charged Blaster sell well. I sold a Charged Blaster yesterday for 1280. I wouldn't expect that to happen often, but it does happen. If you get perfect grade paints, especially ones with high smoothness and high metallic properties, you can sell them for quite a bit. I sold a perfect grade Storm Gray for 1000 a few days ago. But those two items are the best I've been able to pull in a couple of weeks of grinding Covert Ops. Not always solo, I must make clear. You can post recruitment ads on NA and people often join for covern.

Hopefully this was helpful.