There's nothing to stop you using the fx on the 50B multi unit as if the bays were stomp boxes as far as I can tell.
''Some songs that could use some fx'' might well be fully covered with what you have already.
Do that for a start and add other boxes as you feel. Then retire the Boss, or keep it, or give the whole fx thing away.
The only thing worse than having a multi fx unit not being used is having ten pedals next to it.