All very subjective but, if it were you, I’d keep the sage and worry about replacing it when/if it breaks.
If you are not using the Pro-500, you might as well sell it. The way I see it, is that an idle machine is devaluing if not being used. I’d advertise for a price you are happy to let her go. If no one wants to buy her, then keep it.
Since you don’t drink milk based drinks, an HX makes absolutely no sense, so whenever you buy a new machine, think along the sides of a dual boiler (so you can turn the steam boiler off) or a SBDU like the ECM Classika or the Bezzera Unica, so you can have a milk based drink if you fancy one. And of course, there’s the Puristika, but that is purely for espresso.
Grinder wise… if you can single dose your grinder and you are happy, keep it. Otherwise I’d go on the Single Dose path if you switch beans or brew methods frequently.