So as far as I understand, wiring LED lights in parallel or serial will not cause load issues with the smart switches or burn them out unless you had tons if them. Everything however should be wired in parallel (if wired serially a failed bulb would cause all bulbs to fail). To burn out a smart switch, you’d either have a faulty switch or more likely wire something incorrectly.
My guess as stated above is that you wired the smart switches like regular 3-way switches. They need to be wired differently. If you wire them like your existing 3-way dumb switches, you’ll break them. There should be no power from breaker connected to your add-on paddle or into the traveler connection. The add-on traveler should be wired directly to traveler on the smart switch. Then neutral should be connected into the add-on switch. If you connect line or load, you’ll fry the switch(es).
Please read the FAQ