I registered an account and came out of lurk mode to post my thoughts on this headunit. I'm a new 2019 4r owner (two previous tacomas) and picked up the T9 two weeks ago, it's been installed for a little over a week.
The install went ok. There are no instructions anywhere to be found and after getting it crammed into the dash, the backup camera didn't work and the wireless signals for 4G and Wifi were terrible. Turns out that was my fault. The wiring harness is surprisingly involved and I had missed hooking up the backup camera. The wireless signal issues were because the 4G+ connector is actually for the 4G antenna and the "TV" connector is for wifi.
Fit and finish are ok, not factory but not bad at all. The biggest issue I had was some holes just aren't large enough for the screws and needed to be drilled a bit.
With an ATT SIM installed, I get....H+ speeds

I'm not sure if this is ATT being jerks with the broadband APN and not being a known IMEI (they default unknown IMEIs to HSPDA instead of 4G) or if it's a band issue, but it's enough for things like spotify and maps to work.
WIFI is strong, but I have 5 APs in my house including one in the garage, so take that with a grain of salt
Speed is great, I've had no slow downs at all throughout the day. cold boot time is around 30 seconds, coming out of standby around 5 but the backup camera works immediately.
I've had several crashes and resets which turned out to be related to a bad USB thumb drive that would corrupt at random. Once I removed it and replaced it with a better drive, I've had no more weird resets.
The screen is making a terrible high pitched whine. It sounds like an inverter coil being overdriven, I've reached out to the vendor I bought it from on Amazon to see what they suggest. It's easy enough to block it out with music playing but it doesn't matter if the engine is running or not, the sound is coming directly from the screen.
I do infosec for a living so I have a dedicated gmail account assigned to this unit and I'll be digging into it (and watching its network traffic) over the next few weeks.
It has 2 bluetooth radios which can get a little confusing when you try to pair your phone or whatever. It does allow for one radio to be paired to your phone and one to an OBDII adapter (I'm using BlueDriver) without conflict.
Dash controls all work, CAN bus stuff is a little weird. There is a CAN bus setup inside the extra settings menu that I'm toying around with but it doesn't reset when you do a factory reset, so I'm a little hesitant to go too far. I have been able to do little things though like turn the power standby to a day and turn off that silly air conditioning status popup screen. I've also found where to turn off things like video playback if the vehicle is moving.
It plays loud. A lot louder than the factory non JBL+Nav that I had OEM.
Overall, I feel pretty happy for the money I've spent and the upgrade it provides. It's not perfect and I feel like if this had something like lineageOS available to it, it would be a no brainer. Having said that, baring anymore weird stability issues, I'll keep it even if the screen whines.