Congrats on attending a jam session with a diverse set of instruments and singing. I hope you had fun first of all.
If you couldn’t hear your guitar imagine what the ukes and mandolin experienced? Remember - the sound from your acoustic travels out and forwards so it is likely everyone in the room heard it better than you.
@DavidP suggests that gauge 10 on a dreadnought could be worth upping - yes, they are very light and will impact the overall volume.
@Rossco01 mentions room positioning and that is worth considering.
Room acoustics may also play a part but are beyond your control.
And, as @jkahn mentions, strumming with 20mm isn’t going to be very loud.
Another aspect could be finding your ‘sonic space’ in the mix. If your sound is not cutting through then think about using a capo to raise your overall pitch as it may be that your sound is sitting in the mid range which other sounds (the voices / accordion) are filling so is getting lost in the mix. Write the chord charts out to suit a guitar with capo anywhere between frets 4-7 perhaps. See this topic to help with that.