I’m surprised this information isn’t on the web site for Arc and Arc Ultra. 
 

Your One SL’s will be fine as surrounds, and your Sub is also compatible. 

Yup, I thought it would be there:

Which Sonos speakers work as surrounds with Arc Ultra?

A matching pair of the following speakers will work as surrounds: Era 300, Era 100, Sonos One (including SL), Sonos Five, Play:5 (Gen 2), and SYMFONISK (Gen 2) speakers. You can also use Amp to power in-wall, in-ceiling, or other passive speakers as surrounds.

Which generations of Sub are compatible with Arc Ultra?

All generations of Sub and Sub Mini are compatible with Arc Ultra. A dual Sub setup requires Sub 4 and/or Sub (Gen 3).

Excellent to hear this. Thank you.

Now I have to decide on two things.

1. SInce I do not have a newer HDMI Arc TV, is it necessary to pick Arc Ultra over regular Arc since the sound benefits will not appear until I upgrade the TV?

2. If the Arc Ultra standalone is superior to my Playbar with Sub (which I doubt), maybe I should sell Playbar and Sub and replace it with Arc Ultra.

Neven

As you say, @Neven, that’s for you to decide

Regarding question 1, I would consider  that the Arc Ultra is more future proof, being the newer model. Unless you can find an Arc at a steep discount, I think you’re better off getting an Ultra.  Also, there are devices that can convert an HDMI audio signal to HDMI-eARC, like HD Fury Arcana, that eliminates the need for a TV with Arc capabilities.  That said, it may make more sense just to wait an upgrade your TV.

Regarding question 2, Sonos is stating that the bass output  for Ultra is double that of the Arc.  They are not saying that it matches the bass of a sub.  In fact, they are selling Ultra +sub as a bundled package.

When I installed my Arc Ultra I thought the installation process indicated that my Play 5 (gen. 2) speakers could not be configured?  

Were the Play 5 (gen 2)  configured as individual Rooms?

Paired speakers aren’t available to use as surrounds.

Were all Sonos on the same version of the software? Settings, Manage, About and see each Room’s Version: xx.xx-xxxx number.

Ah, That would be the reason as I had the Play 5 (gen. 2) and Sonos Sub (gen. 3) as my first Sonos products prior to owning a sound bar.  When I purchased the Arc Ultra and started the installation process I learned that I could only use my Sub with either my Play 5s or alternatively with my sound bar (but not both).  I had specifically purchased a Sonos sound bar thinking I was ahead of the game by spreading having the Sub.  Only then did I learn I would need to constantly move the Sub back and forth virtually and actually name the new Sound Bar as if it was in a different room (even though they are all in the same room).  So now I have a family room and a living room both actually being the same Great Room.  

Shame Sonos can’t figure out a way to make that less confusing, like calling the Sonos groups Zones and the rooms in your house rooms?

I got a decent deal on a Sub so I have enough to have one on my TV and Atmos music setup. I’d hate to give up either and also hate the hassle of swapping back and forth, not to mention redoing Trueplay after every swap. Used, rough condition looks to be around $400 and might get better as folks want to trade-up to a Sub 4..