There's a reason users/clients have come to fear the Google acquisition of a product they've come to depend on and in which they've vested hopes of continued improvements and solutions.
At least Google hasn't outright put a bullet in Voice, yet.
I'm preparing for that day, when GVoice dies.
There are lots of great XMPP clients, such as Conversations on Android, and Gajim or Adium on desktop. Or use https://movim.eu/ if you like a web client. iOS has a few options as well, including Tigase Messenger and IM+.
Unless you're very careful, they mush together your Voice and Fi accounts, irreversibly.
When Fi was in its early days, a lot of Fi users found this out the hard way and were pissed about it. People who'd been using Voice, maintaining a separate Voice #, and who wanted to keep it that way. Who DIDN'T want that number suddenly tied to a/their cell phone.
(Hint: Create a separate browser profile (not just a Google account, but an actual separate profile under its own sub-directory). Sign up for a different Gmail/Google account. Use one account for Google Voice, and the other for Google Fi. Don't accidentally cross-pollinate.)