Someone from Spirit (Tomas?) who could give their insights on this? Thanks!

I am afraid but Spektrum telemetry receivers are not supported with OpenTX. And if somebody did such hack, we can't guarantee it can work properly.
If you can see corrupted data then please send question to manufacturer of this radio. I guess no Spektrum telemetry receiver is supported there.

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Spirit System developer

Thanks!
What receiver (9ch) and telemetry (RPM/Current/voltage) do you recommend for use with OpenTX integration and Spirit?

Frsky X4R-SB are great for this job.
Tiny, long range and only 2 wires (1 SBus + 1 S.Port for the telemetry).
Depending of your Spirit unit, you may need a SBus inverter cable.

[EDIT] I saw you got a Spirit Pro. So, you don't need an inverter cable. Spirit Pro can be directly plug on the FrSky X4R-SB.

Thanks, went for the RX8R Pro with MLVSS and FAS40S sensors

It should do the job.
Are you sure a 40A current sensor will be enough?
If you are using XT90 connector, the FAS100S sensor could be a good choice too.

I am just hovering and forward flying with my 470LM. The XT90 connector does not fit my XT60 lipos. Tha's the reason I went for the 40A telemetry

You should be good then.

Both the FrSky integration cable as the wifilink cable go into the spirit pro sys port. Does that mean both cannot be used at the same time or can you use a y-lead for using both?

This is a good point. I don't know.
My wiring for my both heli are:
X4R SBus → RUD input of Spirit Pro
X4R SPort → SYS of Spirit Pro using this cable: FrSky integration cable

If I need to plug the WiFi or the USB dongle, I disconnect the "SYS" wire then plug the WiFi/USB dongle.
In my setup I added a very very short servo extension on the SYS port. Thus I can easily do the permutation without disconnecting the wire on the Spirit.
"SYS" Spirit <---> Servo short extension wire <---> FrSky integration cable to the X4R-SB SPort pi
OR
"SYS" Spirit <---> Servo short extension wire <---> WiFi/USB dongle wire

For your FAS40 sensor you can use a Y wire on the X4R-SB SPort.
If you want to use the governor mode (I highly recommend it), you can connect your HobbyWing ESC to the Spirit using this cable : Telem UNI cable
This will also give you the RPM, temp, etc...
More information here: HobbyWing Telemetry

Last point, for 500 class heli, a dual power of the Spirit Unit must be wires.
In my case, 1 power line through the SBus (GND + Power + Signal) + 1 power line directly from the ESC to the Spirit (GND + Power).
Note: Never power the Spirit unit through the SYS port.