Sounds like you're got a great platform already!
I have a .30 600mm and a .25 700mm (STX and slug) set up with harmonic tuner, CF sleeve, and DonnyFL Fat Boy to switch between all calibers. I love how the .30 hits the steel targets but for general fun and tuning I spend most of my time with the .25.
Without knowing your goals (shooting improvement, tweaking, long-range, cheap plinking, hunting, etc), plus how long your home (or nearby) range is for where you can shoot regularly (and wind considerations), it's hard to give informed advice. Additionally, you've not mentioned having an in-house air source (compressor and/or transfer tank), which I consider a must for long-term enjoyment. Driving to fill tanks and hand pumps are a bear. Especially with multiple guns to feed.
Pellet availability is getting better again, with .22 being the king for options, but .25 and .30 also have lots of options, good grain spread, and types.
So...saying all that:
- As I enjoy the tweaking and tuning involved in precision shooting; I would suggest maybe a removable harmonic tuner for your current set-up, and chronograph as an alternative to another barrel. Also, an in-line regulator for your transfer tank is nice to keep from constantly filling the gun's tank (although your 580 is nice and big). With shorter barrels being stiffer, a CF sleeve is better served on a 700mm.
- For cheap plinking, a short .22 barrel for portability/maneuvering in the woods, or long .22 for higher velocity and shot count from the bench.
- I would only go .177 if shot count and your range (neighbors) limit how much energy you want flying downrange.
- For hard hitting against steel targets at distance, .30 is a blast. Plus the heavier pellets do better in the wind, as do .25 slugs. Longer barrels for higher velocities...again from a bench only though.