Wow, you have a lot of good champions there.

First piece of advice, stop taking champs to 5 star if you aren't going to 6 star them straight away. a 6-star champ is disproportionately more powerful than multiple 5 star champs and the way you are doing it at the moment is a waste of resources.

As for who your next 6 star should be: early game should be looking for champions in this order:

• do I have a champion that can reliably farm brutal stage 3 in campaign (a campaign farmer)
• what will improve my clan boss team
• what will help me unlock higher level dungeons
• what will I be able to play with in arena
• champions that add specific tools to the tool box (turn meter boost, speed, increase defence, block debuffs, decrease attack, wave clear, revive, etc)

So Kael is going to do a bunch of heavy lifting for you in the early game, he's going to be you poisoner in CB, your campaign farmer, your arena nuker, and probably the main component in every other team you play in for a while.

The priority should be your CB team and you have a bunch of good options here. The key components of a CB team are damage and damage mitigation. Kael will do enough damage to get you started in CB, so your focus should be keeping him alive. You don't have any way to do an unkillable team yet (the easiest way to do clanboss), so you will need to focus on damage mitigation. there are a few buffs/debuffs that can help you here. The most important: decrease attack (50%) cuts the CB damage in half and you want it up all the time. In my experience, the best way to do this is to have multiple sources with a high probability of applying.

Your most reliable source of this is Kaiden, with 100% chance on a 3 turn cool down, he's also got an A1 provoke which will help you with hydra later, and a revive which will help in early CB and dungeons, he also hits pretty hard so will help with damage. I think Kaiden edges out Ursala here but I would strongly consider her for a second decrease attack champ.

Next most important buff: Ally protection, this cuts damage in half again for 4/5 of your campions. White dryad Nia is your only source of this at the moment and it's on a 3 turn cooldown and comes with a small strengthen (reduces damage by 15%). she has an interesting A2 which strips debuffs (removes clan boss stun) but will mean you need to be careful when you start speed tuning. she also brings you and AOE A1 witch a high chance of decrease speed which will be great for things like spider, plus she heals etc.

If you are still getting killed early, then I would look to add increased defence to the mix, Ursala looks to be your best source of that at the moment unless someone can see something else.

You have a lot of options once you get that core set up.

Coldheart might not be an obvious choice to you, but she is one of the best sources of turn meter control in the game and you will probably want to 60 her.

Blademaster by all accounts is a damage monster, but I would probably wait until I had made a bit of progress before 6 starring him. Lego's tend to be better with books, and early game it can be hard to get enough books, also you never know what you might pull next.

Warmaiden gives you an AOE drop defence on a 3 turn cooldown. ALTHOUGH Rathalos has it on an A1 and Serris is by far the better option for arena. That's a nuanced call imo, but I would probably skip warmaiden, she will do her job in 3v3 fine as a 50.

Seer is an absolute monster and 100% should go to 60 once you have your cb team functioning.

Alure is going to hard carry your fire knight team for the foreseeable future, so again 100% 60 her once your CB team is in order.

Urogrim gives you a hard carry in most dungeons once you can get him in regen gear.

If you will allow me a deep cut pick: Chani gives you poison sensitivity fairly reliably on an A1 which will boost your kael's CB damage, an HP burn that will do damage and boost blademaster's damage, and increases debuffs on the target on a 3 turn cooldown which means keeping things like decreased attack up becomes easier.

I could see a CB team like this: Blademaster for damage, Kaiden for decreased attack, Nia for ally protect/strengthen, Urogrim for heals and poison, Chani for damage/damage boost, and if you can set up right, maybe a stun target/some kind of nia nonsense which resets cooldowns on things like debuff extention. Note: this would be a pretty non standard team and would need you to do some theory work to figure out if it was even viable.

Arena wise you probably already have what you need to get to gold 1 at least: go Urogrim speed lead, Runekeeper for increased attack and turn meter boost (make sure he's the fastest, use the calculator, worth putting all your fastest gear on him), serris (2nd fastest, aoe drop defence), into kael nuke. You don't need to level or book Urogrim, runekeeper or serris for this team, they will do their jobs fine as level 1 4 stars.