I think it depends on what you used the space for.  Your current layout will concentrate the audio to the center space, which is fine if that’s the main listening area.  You will get less volume on the edges.  If you do this setup, I would alternate the left and right channels so that the edges will hear both channels of audio.  Although that will muddy things in the middle a bit, and to be honest, 22’ isn’t that wide and I don’t think you really need speakers on both sides.

If it were me, I probably would put speakers on all 4 posts on one side, alternating left and right channels.  This way, you maintain a sound stage and you have coverage over most of the space.   Although I don’t have a porch like you, this is how I setup speakers in my backyard. Depending on the ceiling/roof,  6 in ceiling speakers would be nice too.  This would be a good option of the space is open without a real listening area, somewhat like you would have in a kitchen, bathroom, or hallway.

@melvimbe, thank you so much -- this is very helpful. Never even occurred to me to only have them on 1 side. Unfortunately though, for reasons, I won’t get into much here now though, I don’t think that’s an option. In short, there is a bunch of other stuff and panels on the ceiling and posts which make locating the speakers tricky.

FYI, we want sound to be solid everywhere as there isn’t a main listening area.

Regarding switching to ceiling speakers, do you think this space needs 6 speakers or is 4 sufficient? If 4, could they be roughly where I showed the Mariners? Or would they need to be spread further apart (to the east and the west) in order to spread the sound more evenly?

Thank you again.

You could get away with 4, but I think 6 for this case.  Essentially because each speaker will need to play a little louder with 4, and it will be louder near the speaker than farther away.  With 6, you will have a bit less volume need to cover the space, and it will sound more even over all.  4 Is fine, just 6 would be better IMO.

For 4 your position is fine, though I might go a little wider. The center area is getting audio from all 4 speakers, while the sides are only getting audio from 2, effectively.  For 6, just space them evenly apart.

Please note that a single Sonos amp can handle 2 pairs of 8 ohm speakers normally, 3 pair if you use the Sonance/Sonos architectural speakers sold on this site. (Of course you can also get a speaker switch to to do the impedance matching too)  As well with the in ceiling or in wall Sonos/Sonance speakers, you can do trueplay tuning.  You can’t do tuning with other speakers or the Sonos/Sonance outdoor speakers.  I have not done this myself, so can’t say how much of a difference it makes.