Bypass with low loss header

If your boiler is cast iron non condensing and a low loss header was installed I would suggest it’s not a by pass you need ,what is needed would be danfoss thermic valve to prevent prolonged low temperature return temps which will possibly cause flue gas condensation and ruin your boiler . A thermic valve is the better choice then a valved by pass which will never really have a idea of what the return temp is while a thermic valve has some what of a brain . In which case a hydro separator is good for hydronic pump separation but a 4 port buffer tank like a boiler buddy would have been a better choice being it would give you some buffering and possible alleviate any short cycling by giving you volume being most radiant systems don’t have a lot of volume and after initially coming up to temp they don’t shed a lot of btu s and w a non modulating boiler short cycling usually starts ,while a buffer tank will store the extra heat and the radiant slips off it until the temp falls low enough to re engage the boiler .
Food for thought
Peace and good luck clammy