Currently, it's mostly manual removals which isn't sustainable if the platform grows. Various instances are experimenting with their own moderation tools outside of Lemmy, and I don't think Lemmy itself has any features to combat this. Moderation improvements is something that's been talked about with Sublinks.

What additional measures could further improve these efforts?

Having an 'automod', similar to but more advanced than Reddit, would help a lot as the first step. No one likes excess use of automod, but not having it at all will be much worse. Having an improved automod system with guides and tips on how to use it effectively, will go a long way towards making moderation easier.

We're not mainstream enough to have many bots yet. I think some instances needed to deal with bot spam, but I haven't seen any in the community I moderate.

By not being popular enough to attract the majority of them

We don't, because bots don't know about us.

As a moderator of a couple communities, some basic/copypasta misbehaviour is caught by automated bots that I largely had to bootstrap or heavily modify myself. Near everything else has to be manually reviewed, which obviously isn't particularly sustainable in the long term.

Improving the situation is a complex issue, since these kinds of tools often require a level of secrecy incompatible with FOSS principles to work effectively. If you publicly publish your model/algorithm for detecting spam, spammers will simply craft their content to avoid it by testing against it. This problem extends to accessing third party tools, such as specialised tools Microsoft and Google provide for identifying and reporting CSAM content to authorities. They are generally unwilling to provision their service to small actors, IMO in an attempt to stop producers themselves testing and manipulating their content to subvert the tool.

Threativore

Using hard af Captchas

it welcome bot

Personally I just block them

You can filter bots that identify as such in your account settings.