It sounds like you have your problem well defined with the turn off of the AC motors on the output side of the relays.
Before changing from relays to SSRs I would explore noise suppression of the motors because you might need that with the SSR as well. You will need to place an RC snubber or MOV across the motor.
This hardware selection is not really my area, and I would just be giving back my internet search findings. I encourage you to spend some time with Mr Google or the forum search box as well. Your motors are pretty small, the 240V is standard and I don't think you will have a problem selecting something or building something yourself.
If after the research you can't decide on what to use, you might also want to make a new posting in the General Electronic section of the forum with your current problem statement, the latest isolated circuit diagram, the motor information, etc and ask for RC snubber or MOV recomendations.
There are other noise suppression things to consider like the distance between the Arduino and the motors. Possibly shielding the Arduino in some sort of metal box, or wrapping running the wires from the motors to the sockets around some ferrite cores which also suppresses transients.
EMI and electrical noise issues from switching inductive loads can be tricky. I think you've done the right things on the primary side. If after you work on the AC side for a bit and your problems don't go away then you will be needing the black magic.