How do I convert an old Satchwell/Sunvic TLX2203 thermostat wiring to line voltage wiring?
Step one would be to confirm the function of the wires -- that might involve looking into the circuit breaker panel and the wiring compartment on the heaters. You might use a volt meter and/or a non-contact voltage detector to confirm which is the hot/live wire and which is the switched live/load wire going to the heater.
Based on the diagram you showed for the Sinope this device does not require a neutral connection. The neutral could be left capped so that it's safe but available for future use if needed. Then the live/supply wire would connect to one wire of the Sinope device as shown by the L1 connection; the switched live/load wire would connect to the other thermostat terminal. It appears this thermostat doesn't care which of its wires is the live and which is the switched output.
It looks as if the cable directly enters the old thermostat without any wiring box inside the wall. The Sinope appears to use pigtails for its connection -- it might not have a built-in junction box as the old thermostat did. In this case you may need to cut in an &quot;old work&quot; junction box to provide wiring space inside the wall. Pictured below is an old work box made by Carlon (photo from Gordon Electric Supply).