Something doesn't make sense. Why do you care about harmonics at the antenna level? Surely your tuner will filter those out anyway. As long as the antenna resonates well and has the right impedance at the desired frequency, why do you care how much it picks up harmonics too? The first tank circuit in the tuner should squash those pretty well.

I guess question #1 here is: is this a simplex or duplex radio? Any antenna switches etc present?

For me, a schematic (or even a basic shema) would help.

We care about the harmonics coming out of the product because they are over the FCC limits! There's no antenna switch, it's a reference design LoRa radio (there's a lump of passive components to tune to 50 ohms out of the chip, and then I've got an empty pi network now to fix these harmonics). I'm just trying to see at a high level how you tackle filtering the harmonics while maintaining the existing matched network. I don't think the solution is super binary.

You originally said this was a radio. Now you say it's a transmitter. Which is it?

Your 6th harmonic are over the spurious emission limits on a LoRa? That sounds strange, did you deviate a lot from reference designs? How many dBm are the harmonics at and what's your carrier output power?