However Ive started to think whether its better to use the R3 R4 relationship and just choose for ex R4 to 2Mohm and then solve, its supposed to be high resistans.
Looks like a source follower. Rd can be zero ohms.
You usually want source to be near mid-supply for max output swing. Actual best swing is when source is supply/2 minus half Vgs. Gate will be half supply plus half Vgs. The value of Rs will set the current through device. Add Vgs to Vs to set the R3/R4 divider. JFET will have very small, nanoamps, of gate current so don't have to worry too much about gate bias current.