Yeah, definitely not Saurus. Your two options basically are Kael or Tayrel. The traditional choice is to take your starter, so Kael, but since you got lucky and had Tayrel from a shard, you could also take him instead, as he's a better long term choice. And to be clear, after you take Tayrel/Kael to 5*, your next step is to take them to 6*, which will obviously take a lot more grinding still - but you could do the grinding at a higher stage then. 

Based on your current roster, if you take Tayrel to 6* first, you'd probably still take Kael second, but perhaps by the time you get there, you will have enough better champions that you might just leave Kael at 5*, basically skipping the stage of the game where you use him everywhere. Perhaps also not. You really need a poisoner for the Clan Boss, so until you get one of the champions from the fairly short list of poisoning specialists, you'd still use Kael for that. 

By the way, are any of the 4* "just good for food"? So I could use them as base for 6* progression? :)

No, I wouldn't say so... Doompriest is by far the best of them after Tayrel and Kael. Hope has some potential as support, Warden is also not too bad (reflect damage works well on fire knight). Neither of the barbarians is great exactly, but both have decrease defense and weaken, so you'll probably want to take one of the two on your CB team, at least for the time being, as well as other places perhaps. 

Kael

Kael to 60, tayrel and doompriest (doesnt need books) next

Kael first for campaign farming, then Tayrel for all around utility. 

If you pull an Apothecary, rank him up after Kael and Tayrel. 

Saurus sounds like a great idea, but whatever gear you put on him to be a fast farmer will be better used on more versatile champs.  I'd leave at 40.

You have great early pulls from shards, I'd say you are 1 champ away from a team that can progress all the way to dragon20!  

1. Kael to 60

2. Save gems (don't buy refills, don't buy shards)

3. Spend 800 gems on masteries for Kael