KFLOP Outputs (the ones without pull down resistors) can operate in "open collector" mode to drive most "5V devices".
See this diagram:
The situation is somewhat complicated by the internal protection diodes KFLOP has connected to the 3.3V supply.
When the output sinks low (switch closed) >4.5V @ 16ma can be applied to the load. This normally meets the specification.
When "open" (switch open) then the difference between 5V and 3.3V less one diode drop is applied to the load (~1.2V). Often the maximum allowed voltage for the device to be off is not specified. Or specified to be 0.5V or less. Since most inputs involve an LED that only begins to turn on at 1.4V the interface will work.
Some SSR are specified to turn on at 3V. These would be a better choice and could be connected to the 3.3V supply and the device would see 0V when turned off.
So if well understood I can buy an SSR that has a specific input lower limit of 3V and can connect the +ve side to pin 5 and the -ve to pin 8 of JP4 directly without any buffer circuits/or any additional diodes/resistors.
The first 8 IO on KFLOP JP4 and JP6 have relatively strong pull down resistors (150ohms). Because of this pin 5 is only guaranteed to have a high value of 2.4V and will not source much current. Therefore it wouldn't meet the specification of 3V.
The last 2 IO (pins15 and 16) should work in that manner. Or the IO on JP7 or JP5.