Move to a cheap 2 channel or single channel VHF system. Virtually no interference in the below 300 mhz band. The only problem is these systems usually come with cheaper lav mics included. You will have great signal strength, but low sound quality. I have had minimal issues with my $80 NADY DKW set. Sound quality is just sub par to fair though. Tinty sound.
Either that, or go all out, and spend $1,000+ on a Shure QLX system. Everything in the middle is going to have their share of issues you will eventually contend with when it comes to lavalier microphones. None of them will be "Rock Solid". SHURE BLX sounds great, plenty of pick up, but short signal drop outs occur on multiple channels, and if the officiant is moving around a lot, it's not great at keeping the signal going.
My cheap EBM $34 dual set of lav mics have great signal stregnth. BAD on pick up/volume strength. Quality of speech is actually perhaps a bit better than the NADY $80 set, but you need to literally be holding the mic element 3 to 4 inches from mouth, OR really push that mic gain way up for them to work fairly.