Upgrade to the Beam Ultra, the Beam gen 1 can move to another TV or it is a pretty decent stand-alone speaker for another room. Worst case gift or sell it.
Play 5s aren't great as surrounds, the sound is great, but compared to the cost it doesn't pay back well. The only exception is if you do a lot of streaming in Full Mode and want the Room really full of sound but with poor stereo imaging.
Upgrading the Beam and moving the 300s to surround duty will make a huge improvement to your home theaters sound, particularly Atmos and free up the Play 5s for use where they would be used closer to their full abilities, two mono Rooms or as a stereo Pair.
If you are going to have access to Atmos music adding more 300s as an Atmos Pair would be really tempting. Again, relocate the Play 5s where they make the most sense.