question

Unfortunately, the product page for the 709JS sender does not offer any information at all about how it works, so it's hard to say for sure what it would work with. By looking at the picture of it, it looks like it is not compatible, but looks can be deceiving.
The Cerbo requires resistive senders, so that is the fundamental parameter that you are looking for, if the manufacturer is good enough to provide such information.

Thank you for the reply. I will look for another option but in the meantime I successfully installed two resistive senders in my two potable H20 tanks - it is very nice to know how much fresh water I have onboard.

Have a look at the following holding tank sensor from Kus - it may not be exactly what you are after but might be helpful in your search https://kus-usa.com/product/holding-tank-level-sender-shs-shst/

I just posted this elsewhere but it is relevant to this thread too.

It would be nice if the GX Tank 140 could interface with SeeLeveL senders but, according to Garnet Instruments, their own display/interface is required. In fact, their display/interface eliminates the need for the GX Tank 140, connecting directly to the Cerbo GX. Personally, I would prefer to use the GX Tank 140 since the rest of my system is Victron and the GX Tank 140 is powered by a simple USB connection, but the absence of any directory of compatible sensors makes it hard to recommend. The compatible senders I've found on my own either require drilling into the tank or access to the bottom of it. The SeeLeveL senders adhere to the side of a tank (plastic only) which makes for easy installation for anyone who has access to the side of their water tank.

It's worth noting that not every See LeveL display/interface is compatible with the Cerbo GX. It has to have a data output. Also note that the See LeveL interfaces that do have an output use one of two different protocols. The Cerbo GX can communicate using either protocol but all VE.Can ports on the Cerbo have to use the *same* protocol. If you've got other devices connected to the Cerbo's VE.Can ports, you must choose the See LeveL interface version that uses the same protocol.