I would point out that, historically, these trade-offs have been well-trod, going back to 1990's usenet and email, & IRC. Email spamblock developed in layers: legal action against the most ergregious spam cannons, server-side filtering with realtime blackhole lists, and client side with beyesian and deterministic rules. POW, of course, was first proposed for email but never caught on, but we do have it for NOSTR. Usenet had the concept of "bozo filters" for client-side filtering. IRC, in its early days, had the "eris" server that refused on principle to filter anything, but eventually everyone else stopped federating and EFNet (Eris-free) is now the oldest extant IRC network.
As a general rule, server-side filtering creates a hook for authoritarian censorship, which defeats the whole point of NOSTR, so that is a last resort only to be considered when a client-side is truly impossible.
NOSTR support for POW gives us, in my view, the magic bullet to avoid email's fate. Though email is still quite nice if you take a half hour every month or so to tweak your client's filtering rules, that is not a grandma-level skill. I predict that, as NOSTR's popularity rises, spam will get worse, and eventually everyone will start setting POW thresholds. When grandma gets crapflooded, you just can tell her to bump up her minumum POW and life will go merrily along.
