that i7 6700k and gtx 1070 was an absolute legendary combo back in the day but your whole system is on a completely dead platform right now. you cannot just upgrade one part without heavily bottlenecking the rest of the pc.

with 700 euros you are basically going to be building a brand new core inside your old case. since you do rendering you definitely need to stick with nvidia for the cuda cores and you want a cpu with some decent multi threading.

here is the exact play to maximize that budget

first grab a ryzen 7 5700x and a budget b550 motherboard. that gets you 8 solid cores for rendering and it is insanely cheap right now.

for memory just reuse your current 16gb ddr4 sticks for now. with the massive global ram shortage going on right now buying new ddr4 or moving to a ddr5 am5 build will completely blow your 700 euro budget.

take all the money you saved there and hunt down a used rtx 3070 ti or a used rtx 4060 ti 16gb. they will absolutely crush any modern game at 1080p or 1440p and handle your rendering projects with zero sweat.

just make sure you double check your power supply before you buy anything. if your current unit is under 600w or over six years old you really should squeeze a decent 650w unit into that budget so your new parts do not randomly shut down when you start rendering.