Yep, the con is a big con. Maybe the best way to fix that would be to have a 'compiled' meme: generate as an alternate an old style image version of the meme and send it only to old clients, other Fediverse server software and old Mastodon servers. It would break pro #2, but on time even that crutch could be removed.
@smitten
Yes. If SVG is supported by all software, you could embed the base bitmap as an external reference in the SVG, letting it store only the text and any metadata that could be useful, as SVG allows the use of custom tags and attributes. So you would not need to use it as an alternative; it could be the new meme format in itself, because it would cover all your initial points.
I'm not familiar with how images work in native apps but I imagine it could take that URL too.