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.