Potential System Upgrade

Depending upon what you want to upgrade and the size/layout of your rooms, my first thought is a second Era 300 to make a stereo pair for your records. Streamed Atmos music will sound pretty good too through two Era 300’s. Add a Sub Mini to that room when you have the budget. Or a Sub/Sub Mini if your home theatre system needs more emphasis. 

Watch for a sale and add the second 300 for Atmos if you nave an Atmos source and if possible the Sub-mini. 

If no Atmos then the mini Sub for the Beam.

My questions are…

Are you wanting to upgrade to stream music or for watching movies?

Is Dolby Atmos important?

If you add a 300 and use the pair for surround you’ll need to move the record player to the 100’s using the USB adapter.

Scroll down to the FAQ, I don't think the 300 and Gen 1 Beam are supported.

https://www.sonos.com/en-us/shop/era-300

Because Era 300 is a multichannel rear speaker, it requires a Sonos soundbar that supports Dolby Atmos, i.e. Arc or Beam (Gen 2). For this reason, Era 300 is not compatible with Playbar, Playbase, or Beam (Gen 1).

As other users have said to you, it’s important to know your priorities. 

Before to improve the Beam with other Beam, I was thinking to recommend you to go to the previous Arc, cheaper than the new Ultra, but it has disappeared from Amazon and Sonos is showing it without any discount in the web, so I think it’s over.

Then, I think the options are clear, go for the second 300 to improve the music quality, or for a Sub mini to enhance the movies. Or wait, save more, and upgrade the TV set with an Arc Ultra in the short future. It depends on your priorities and the sixes of the rooms.