tl;dr- Go Linker. Psycho if you plan on PvP and Thaum if you find yourself dying a lot.
C3 Thaum reporting in.
As a filler you have a few choices but I would limit them to Linker, Thaum, or Psycho.
But it really depends on how you play and what your goal is.
Don’t take C1 Thaum for the buff. That is silly. If it is for the buff alone then your circle will be wasted in every party instance with another Thaum (even if you have maxed attributes chances are, in time, they will to, so you will not have been needed). C1 Thaum is for Transpose. If you find that, even with summons out, you are dying a lot, then Thaum may be a good pick (if you currently have more INT than CON; if you have more CON than INT you could use it to pick up some more damage for your Pyro Skills but likely not a lot). CON adds not only HP but HP Recovery, Block, and Crit Resistance, making Transpose useful for not only the HP pool but also for getting out of a tight spot with additional block etc. Additionally your Thaum buffs will not help your summons. Being able to shrink or swell enemies is also a very unique trait that you may enjoy.
Psycho would be a better pick if you plan on PvP and already have a solid stat build. Psycho brings CC to the table and you’ll be able to keep your enemy pinned down while your summons destroy them. Additionally you have get out of jail cards by being able to teleport away, swap locations with the player (into your ground based AoE’s, i.e. drop a few fireballs etc on top of you and then swap places with them), and more. It’s a very versatile class that shines with Sorc. However it is better with Cryo than Pyro.
Linker would be the filler I would pick for your build. You have a lot of AoE and with your summons you will melt through a pull. Additionally, unlike Thaum where buffs can be overwritten or useless when another higher circle Thaum is in the party, a Linker will always be useful. You could be in a party with 2 or 3 other Linkers of varying levels but it won’t matter; your circle into Linker will still be useful. Not only will it help with your solo play but it will help as well with party play. Anyone who has played with a Linker in the party will be able to see the differences in clear speeds. Also worth mentioning is that even on bosses you can use Linker/Pyro effectively by linking the boss to multiple fireballs and then knot if the boss moves away from them. All around though I feel Linker would be best suited to your build.