Can I use only two sensors? I would choose your SRT300 or 400 but do not like that the min/max cannot be set different for f / rear tire? Are the internal sensors compatible with the SRTP 300 to upgrade?

If you only use 2 sensors, the system will keep beeping, since one sensor will be zero reading.
We have upgraded our SRTP300 recently with a new FW. You are now able to set the front and rear wheels at different pressure.

How long does the battery last in the internal sensor?

The battery life is about 2 years.

Do the internal sensors have a replaceable battery?

The batteries in internal sensors are not replaceable

Can you just buy the sensors and use app?

The sensors are not Bluetooth , cannot use with the APP. The only APP based for 3 wheelers is SRTP630

Will these sensors fit in aluminum wheels on a 2023 Harley Tri-Glide with 8mm opening? Is the stem long enough to go through the wheel?

yes, 8mm is a standard size. The clearance for wheel thickness is 1/4".

What type of mount does it come with?

Comes with a metal handelbar mount and a flat mount

Will the stems fit all three tires on a 2022 Triglide ultra

The stems for this model is  8mm . some wheels use 12mm. You will need to consult your motorcycle dealer, or measure the thickness of your current stems. If 12mm, you can use adapters. If buying SRTP690 direct from SYKIK.com, you can request for adapters at no charge.