i would argue the only downside to client-side rendering is complexity.
you just admitted that the SEO is not an issue if implemented correctly. also, we can easily add server-side rendering to client-side, not so much the other way around. this can easily fix any SEO issues and even offer an alternative method for loading in weak devices or something. i mean, <NOSCRIPT> is still a thing.
initial loading issues is kinda laughable. we can implement it to be even faster than a static page. it is, of course, more complex.
and requiring an external library isn’t a con. you probably meant bigger maintenance. again, more complexity. but this isn’t even necessarily a bad thing: websites should be kept up to date. this kind of enforces it more as well.
anyway, nice article!