Wivre Hairpins are a fantastic synth post 80, it'll slow down past the 85 synth cap but the Wivre in Abyssea drop the required Wivre Mauls like candy, especially with TH.
After that there is Raaz Arrowheads with a 92 cap
Mammoth Tusk 94
Chronos Tooth 100
Those 2 come from Dynamis drops of Giant Frozen Head and Colossal Skull respectively, you won't really be able to farm these effectively as they have a low drop rate but a good number should regularly pop up on the AH.
After those, I would argue the only feasible synth to 110 is Maliyakaleya Orb with a cap of 105, thankfully you can now take that to 110, spare a thought for the bonecrafters who had to finish up doing grossly expensive synths like Tati Earrings.