The relays should be alright as the pwm transistor would shut off the current whilst the relay switched then turn it back on once it has switched. This avoids sparking/arcing which is how relays normally fail, once closed they can take a lot more than their rated current.
I had noticed that a MOSFET was more suitable for low voltages than an IGBT but my issue is with connecting the mosfets, they all seem to have such tiny pins which are hard to connect a large wire to, IGBTs seem to be larger packages which are easier to connect to and you seem to only need one rather than having to connect a bunch in parallel. I am after simplicity but high current handling. Maybe the advantages of the IGBT aren't worth it though.