You could do local multiplayer (Pass and play), upload to steam, abuse the "remote play together" feature and boom "online multiplayer"
In all honesty, your game could do pretty well on there, especially if you keep cooking on it.
Thank you for even considering online multiplayer. That sounds like an absolute nightmare to program and mantain costs.
Wait, what about "Peer to peer" multiplayer?
You know what? I should shut up. It's your game after all.
I hope your game keeps going well and goes strong.