Honestly, given that you have Uriel, I think you should main fire instead. Uriel AW is awesome and trounces anything that dark has. I would awaken her first and switch over to fire main as soon as your grid is upgraded.
Sol AW is good, but cannot carry a light team by herself. Uriel is a core fire hime and can be excellent on her own even if you don't pull the other excellent fire himes (namely Svarog and BFM Amon.) If you're willing to spend one to two mtix, I would actually main light, if you want to remain completely f2p, fire is your best bet imo.
In the meantime, while you're upgrading your fire or light grid, I would do the following for your dark team:
Don't use Nyarlathotep with Satan, both of their def down are A frame. I would also use Manes instead of Sol for heal assuming your grid is SLvl-ed. Manes is pretty fast for an SR, and between her and Satan, you should be able to FB noticeably earlier when using Arthur as soul. If you want to run Andro, you shouldn't need Sol in addition for heal, and Andro can Ex cleanse or CM as needed (well, unless your strategy is to stay alive as long as possible so Andro can do raid-wide heal, I do that a lot with one of my sub accounts. Your dmg output takes a big hit with that strategy though and it doesn't make sense for solo content.)I'm running Satan, Ereshkigal, Hypnos, and Nyarla/Manes/Sol depending on the situation.
Also, light rst don't stack, since they're in the same frame, so you need to stagger Ereshkigal's and Hypnos' light rst for maximum effect.
I would recommend unlocking Mordred. BP + Satan's orb eat is a pretty potent combo. BP is just useful in general even if you no longer main dark.For my Soul, Andromeda, sometimes Arthur.
Beezlebub is really good, you need to know what you're doing though. Rampaging prevents your team from using abilities and bursting, so the standard way to use her is to use up everyone's abilities early in the battle (I usually use everything up on T1 except heal for this strategy), cast rampaging, do stupid dmg for 2T, then hopefully FB soon after that. What makes rampaging really powerful is that it is a separate multiplier, so the 50% literally means 50% more dmg, as opposed to the 15% atk buff, which is only a 7% dmg increase if you're at 117% assault value. It's best to stack all your buffs when rampaging for maximum effect. If you Ex cleanse, use it to cleanse the def down from Beezlebub's second ability before you go on a rampage (you can also use Sol, but then you waste a heal.) The precaution you need to take is that you need to be strong enough to withstand RNG for 2T since Sol or Andro or whoever can't bail you out until the rampaging is over (usually just means you need to go in with high enough hp.)I heard Beezlebub is a decent SR but I don't use her often.
Rangda works similarly but doesn't rampage the whole team. She's good if you're up against debuff resistant stuff. I would use her instead of Ereshkigal unless you're going full turtle.
If you're willing to mtix, what Dejnov says is actually true and not merely a joke (though I am also known to be biased towards light, so... feel free to get a second opinion.)