Yeah, I considered suggesting Habranthus or Pygmy Water Lily (both would add blunt/pierce/magic coverage, Hab would add defensive buffs like one-time negate while Pygmy would add offensive buffs like crit rate/damage and +damage based on the number of attack types in the squad) but like you noticed it'd take a lot of extra work to guarantee Sparaxis can hit all four types.
There seems to be a way to get Sparaxis able to always hit three types without using FM slots though (provided the way type coverage works now doesn't mess things up):
- First spot goes to Sparaxis since you can't build around a Knight if they're not in the squad.
- Second spot goes to Hab or Pygmy for their blunt/pierce/magic coverage for three along with whichever set of abilities you want more.
- Third spot goes to Saffron or Pumpkin since both have magic coverage for 5 (three wasted magic coverage slots but guarantees Spar has magic coverage). Saffron leans defensive in terms of squad buffs while Pumpkin leans offensive.
- Fourth spot goes to a pierce Knight who can fill in Spar's weak spots like skill proc, crit chance and crit rate. Loquat would be a good gap filler since she has scaling skill proc, +skill damage and both crit bonuses.
- Last spot goes to a blunt Knight who can also help with skill proc and crit bonuses. Walnut covers all of the same bases as Loquat so that's more skill proc/damage and more crit rate/damage.
So, Sparaxis should have magic and slash coverage at a bare minimum and, depending on who is picked in the third spot, also have either guaranteed pierce coverage (Saffron) or guaranteed blunt coverage (Pumpkin) with a coin flip chance of getting the fourth type.