If I wanted to buy a block for speculation (thus helping accelerate ipv6), would it need to be crazy large to even be worth it? I imagine the buyers are less interested in 4000 ips here, 200 ips there, right? Like they’ll want /16, /8, etc?
IPv4 sells for ~$40/IP right now.
The smallest block you can buy that is Internet routable is a /24.
If you're buying, you're likely buying from another speculator, so you're not helping accelerate anything, you're simply a(nother) middle man in a (series of) sale(s) of a commodity, looking to profit until the block eventually gets sold to a user.
None of that is said with any judgement, mind, as I've traded a /22 of IPv4 space for quite a handsome profit over the last few years. Just don't pretend there's any altruism or benefit to anyone else from your speculative activities.
Any other purchase reason is likely to result in ARIN pulling your "ownership" entirely when they discover it.
From what I understand most of what's being sold off right now on ipv4 auctions are from companies who had too much IPV4 that they no longer need, or companies that were liquidated.