

Just to clarify for OP:
The format is always SVG, as it’s an open standard. Inkscape is the leading FOSS SVG vector editor. Boxy is a web-based SVG editor which is freemium.
SVG is the W3C standard for vector graphics that can be rendered by practically every browser. Gzip compressed SVG files (svgz) are much smaller but enjoy less support.
Illustrator can import, edit, and export SVG files.
I use snapcast throughout my house and devices, but there’s no snap_server_ for Android.
I’ve been meaning to try roc, for which there is an Android client that will both play and serve.
Sonobus also claims to be many:many; I haven’t tried it either and it doesn’t look particularly active.
I don’t use UPnP or DLNA because of the security issues, so I can’t offer a suggestion about that. I thought DLNA was a pull oriented protocol - like, to send music from your phone you’d have to select and play on your computer with a DLNA client. Can you push media with DLNA?