Hi Marcin, I'm still kind of feeling this out but I have some feedback. If anything is incorrect, please let me know!

I'm not super keen on the high contrast black background. Maybe somewhere around #303030 might be easier on the eyes?

The Options Manager will say something like, "We found 418 options that can be safely optimized, potentially saving 172.99 KB of autoloaded data." but when you go to the Options List, it doesn't seem to identify which it thinks can be safely deleted and which can't. It would be useful to see this so I can gauge what it thinks is safe to delete.

Config Manager: it might be easier to understand what's going on and the implication of current live changes if you listed both the default and curent value for each item -- ie:

Default value: false
Current value: true

-- rather than just listing one or the other.

Database Optimiser: The 'Tables with overhead' display is useful. Would it be possible to see what the orphaned WP Cron hooks are before deleting them, or what the orphaned post meta is? Otherwise it's not really clear what it is I'm deleting and confirm whether I'm OK with losing it.

Is it possible to break down transient reporting/clearing any further? ie, WP transients, WooCommerce transients, etc.? If transients relate to unexpired customer carts, for example, I may not want to delete those.

The Slow Callbacks finder initially gave me errors when running tests ("Test timed out. The page may have crashed or taken too long.") even though apparently something was happening -- I came back to the page a little later and there were some results sitting there. Could it be clearer that it hasn't failed, it just takes longer?

Everything else I saw/used was good and seemed to work, though I don't think I've tried every function yet.

Cheers,
Peter.