So, according to Jason Evangelho (Thunderbird team), when I asked for an ETA:
Currently, we're shooting for after Thunderbird 115 releases. It could either be late Summer or very early fall. Once we get the Account Autodiscovery module from Thunderbird desktop integrated, we'll be pretty close to flipping the switch. And we'll be sure to keep everyone posted on our blog at https://blog.thunderbird.net
I've gotten used to it, but I'm not a massive email user on my phone, mainly use it for alerts so I know to go chek the PC when I get home from work etc.
You can use k9 right now. Thunderbird mobile is going to be just that with a new name. It already received lots of updates because the main dev is working full time on it now
That said, K9 was my favourite android mail client, and I used it up until recently. It was good but always felt like it needed a little extra love to help with bugs. So I think they'll end up with a good product.
I already use it on my desktop, so I am excited for the branded mobile version. Just hoping I can get exchange support on it in some way. I use an extension on the desktop version.
I am, but mostly for the refreshed visuals. Over that happens I'll move the accounts I use K-9 for. After a few months if it works well I'll migrate other accounts. Not sure if I'll bother moving my Gmail and Outlook/Live accounts. I only use Thunderbird desktop to have a local copy.
Technically, I'm already using it as my daily driver (as K-9 Mail) and so have I been for several years already. What I'm looking forward to, is to get parity with desktop Thunderbird.