I didn't think of grassy seed, that could definitely be good.
HDB is moreso you can come in more often against anything, don't forget ditto has basically infinite pp so any recovery on the opposing team it can use. For example, corv gets pressure used against it. It isn't the best option, but it will be the most consistent option.
Although surviving to proc things could be an issue, a lot of things boost their defense's as well, such as Zama and volc. This means volc will only be a little bit worse defensively against them, and they likely have chip so it would be about even. Against something like moon/gambit, ditto ain't doing shit.
Another thing that really hurts ditto is the prevalence of priority, which means it's speed stat doesn't matter in avoiding hits as much. Maybe it could work on psychic terrain teams with psychic seed or scarf? That solves the priority issue.
So you mean like a PP stall? I didn't think of that, but HBD Ditto actually might be good on stall teams.
I assumed Volc wins because it has heals and a higher HP stat. Also, the Tera cannot be copied. So if it has any Tera Blast set, Ditto might not be able to use it the same way. Volc is one of the mons mons reliant on TB. The chances it can hit you harder might be high. Tera Water is particularly problematic. And unless you can find away to safely bring Ditto in multiple times against it, Volc also has more PP for heals. You'd probably have to sac a mon or something?
The problems with Psychic Seed is that Special Defense is far less useful for Ditto in most cases. Volc is one of the few special sweepers, with the rest mostly just being Stored Power types. Powering up their stored power to offset the special defense boost? Probably not too effective. You could Tera Dark to stop Stored Power, but you don't need Psychic Seed for that. The singular special defense boost probably isn't too impactful against Quiver Dance or Calm Mind users anyhow.
Choice Scarf Ditto on Psychic Terrain runs into the same Booster Energy issue. You just lose against BE speed. Basically, fast BE mons are death to PT teams. Choice Scarf Ditto takes up a slot as speed control that isn't good enough speed control. I have this same issue with many different mons on Psychic Terrain teams, including BE Valiant. Anything that was faster just wins against you. In that specific case, it was most commonly Roaring Moon, Iron Boulder, and Barraskewda.
Priority is absolutely an issue for Ditto. This is actually why I though Grassy Seed might be better. Most of the priority in the tier is still physical. Most of the setup sweepers you see are physical. I think you take the matchups Ditto can win and let the teammates handle the ones it can't. But to be honest, I haven't tested Grassy Seed Ditto nearly enough to know if it can win enough important matchups this way. That said, Grassy Seed + Terra Steel would probably have pretty good survivability for a Ditto. You can resist Grassy Glide, E-Speed, Bullet Punch, and Ice Shard. You resist Stored Power as well.
I think Focus Sash is the only other item we discussed that could potentially help with priority. Psychic Seed against Bolt is so niche that I would simply rather leave that to teammates.