Uugo is a great epic not just FW oO

perhaps she doesnt fit your team well but she is great in nearly all places

I see no need for a 2nd maus mage if you got Doompriest.  Zargala and rearguard are worth 60 if you need them in fw. but Uugo seems to be the best investment here

for lego I'd keep the books for the next fusion in 2 weeks, that dwarf looks monstrous

I cant really tell you who to 6 star next but maybe it will help you if I tell you who you will certainly not regret 6 starring in the long run.

All Legos - this one I will make short. I dont think there is a single Lego in the game that you would not want to 6 Star at some point. Once the fusion event is done, you should wait and see how Yoshi performs and decide then if he is worth going first.

Uugo: insane champ! her A2 is bonkers with def down and block buffs and I think there is no other champ who has both those debuffs on one skill. She is obviously great for FW and Arena but she is fully capable of carrying your team anywhere in the game. Dont underestimate that she basically guarantees heals on your team, which is great for Doom Tower freeze boss for example. Also, I dont know of any Champion, be it Epic or Lego that could make her completely "obsolete" the way for example a war maiden get obsolete the moment you pull a better def down champ.

Zargala: while not on the same level as Uugo in my opinion, Zargala still has her uses here and there. She has weaken and def down which is always good and her A2 leads directly to her A3. I think she lost a little value over time but still a champ that I would not regret getting to 6 Stars.

the rest I would consider as "depends". None of them is a bad champ but they can be replaced more or less "easier". Armiger is great as long as you dont have enough Coldhearts or Royal Guards let alone Septimus but only as long as you dont have those. Rearguard Sergeant is near "essential" for Dwarf FW but there are other champs doing what she does elsewhere and same goes for Steelskull to some extend. Kael I would not bother with because you seem to be quite advanced and his uses are "limited" at this point.

Quargan and Vogoth I have no experience with so cant really tell you much about them.

I hope that helps you find a decision!