I'm a newbie too so take this with the understanding that there's probably a better way to do things :) It depends on your colour scheme, if you're able to set one of the RBG values above 1 then having a
glow_hdr_thresold = 1 on your WorldEnvironment will only apply the glow to those objects with R|G|B > 1.
I kinda wish the alpha channel could be set above 1 (or another generic property tested for HDR threshold) for this purpose, but that doesn't seem possible, would love to be corrected!
The other option I see (probably the right way to do it) is a custom ShaderMaterial to apply a glow effect, there are plenty of good examples online.
Depending on the use case, creating alternate sprites with a glow effect and swapping those as needed is simplest.