Sure! Here are a few initial thoughts:

• First, congrats again! The basic building blocks – Webfinger, AS2 rendering and serving of actors and activities and objects, outbound and inbound HTTP sigs, outbound inbox delivery – all seem to be working. Awesome!
• Your Mention tags also look good. I know you're probably not doing mentions of external fediverse accounts yet, but when you do, do you know if you'll include the fully qualified webfinger handle (eg @user@mastodon.social) in the name field? I'm honestly not sure if it's required for interop or not. Some gory details in https://github.com/snarfed/bridgy-fed/issues/493
• One big ergonomic improvement you could make would be to serve (or redirect to) AS2 on your user-facing profile and post URLs, eg https://www.threads.net/@0xjessel , at least for AS2 conneg URLs, as well as on your AP-specific URLs. People often paste remote profile and post URLs into their instance's search box in order to follow or interact with them, which right now doesn't work with your user-facing URLs.
• An alternative to that is to add a <link rel="alternate" type="application/activity+json"> to profile and post HTML pages that points to the AP URL. Details: https://fed.brid.gy/docs#searchable . Not quite as widely supported as serving/redirecting to AS2 on conneg, but can still help.
• The other side of this coin is, if a request for an AP URL doesn't include AS2 conneg, eg it's a user i na browser, consider redirecting to the user-facing URL. This is uncommon, and you all are correctly including the user-facing URL in the url field, but it's still good practice and good UX when interop goes sideways and users get linked to id instead of url.
• You've probably already seen the threads.net vs www.threads.net ambiguity in interop in some fediverse servers. Some servers will see and show your users as @threads.net, some as @www.threads.net, depending on which domain/host they prefer and from where. This is currently complicated, and I don't have clear advice for you yet, but we in the fediverse should figure out clearer guidance on this.
• Your AP profile and post id URLs currently include username, which makes sense until a user changes their username. Much of the fediverse has the same problem, but some servers like Misskey have started switching to opaque ids to allow username changes. Those will still change a user's handle, eg @before@threads.net => @after@threads.net, but at least their AP id stays the same, which is helpful.
• It's great that you're serving outbox and followers/ing collections with real counts! The next step would be to actually serve posts and profiles in them. Some fediverse servers have started using eg outbox to backfill posts when they first see a new remote user.
• Finally, consider delivering self-replies (eg for multi-post threads) to followers. This makes multi-post threads more easily accessible and viewable on remote instances.

Phew! Apologies for the spew here, hope at least some of it is helpful. Most of us are really excited for you all, and we've had fun kicking the tires so far. Happy to answer any q's. Good luck!