rio2

It looks like the gigabyte board you mentioned supports some Xeon E3 and usually these can be had for cheaper than a comporable i7.
Copied from the Gigabyte site:
The ones with N/A in 3rd column do not include iGPUs. Check the specs of these chips on ark.intel.com, see if you can find one you like used for cheaper.
Intel Xeon E3-1275 3.40GHz 8MB 1050 MHz Sandy Bridge 32nm D2 95W 100 F2 Intel Xeon E3-1245 3.30GHz 8MB 1050 MHz Sandy Bridge 32nm D2 95W 100 F2 Intel Xeon E3-1235 3.20GHz 8MB 1050 MHz Sandy Bridge 32nm D2 95W 100 F2 Intel Xeon E3-1225 3.10GHz 6MB 1050 MHz Sandy Bridge 32nm D2 95W 100 F2 Intel Xeon E3-1290 3.60GHz 8MB N/A Sandy Bridge 32nm D2 95W 100 F2 Intel Xeon E3-1280 3.50GHz 8MB N/A Sandy Bridge 32nm D2 95W 100 F2 Intel Xeon E3-1270 3.40GHz 8MB N/A Sandy Bridge 32nm D2 80W 100 F2 Intel Xeon E3-1240 3.30GHz 8MB N/A Sandy Bridge 32nm D2 80W 100 F2 Intel Xeon E3-1230 3.20GHz 8MB N/A Sandy Bridge 32nm D2 80W 100 F2 Intel Xeon E3-1220L 2.20GHz 3MB N/A Sandy Bridge 32nm D2 20W 100 F2 Intel Xeon E3-1220 3.10GHz 8MB N/A Sandy Bridge 32nm D2 80W 100 F2

9th gen is not affected by ZombieLoad, you will be just fine.

The temporary GUI freezing during OCCT is normal, because it takes up all the system resources and leave none to the GUI and input handling, but about about the throttling I honestly have no idea, but measuring the VRM temperatures might be worth giving a shot.

Ryzen will perform way better in your usecase.

Yes the 1700 system and in the end I went with a Corsair RM750x, because I found it on sale for just ~15$ more than RM650x which for some reason costs exactly the same as RM550x in most stores here.

PCPartPicker: https://pcpartpicker.com/list/v8dkRG

CPU    
AMD - Ryzen 7 1700X 3.4GHz 8-Core Processor
$289.87

Motherboard    
ASRock - X370 KILLER SLI/ac ATX AM4 Motherboard
$79.99
Total: $369.86  
And if you don't have an AM4 bracket for your cooler, you can get one free from be quite!'s website.

First try fine tuning the voltage to as low as possible at 5Ghz and check how low can you get those temps, than go down in 100MHz increments and fine tune the voltage at each to the lowest possible voltage at that clock. Do this until your temps don't go over 80C under OCCT, PRIME95 or AIDA64 streess fpu.

Ryzen isn't that much behind Skylake(/Kaby Lake/ Coffee Lake) in 1T andis almost in stock everywhere, but the downside is you actually need a dedicated GPU to use it.
So for example for the price of your 7700k build you can easily get an X370 board, 16GB of RAM and a Ryzen 7 1800x and a cooler, which is way faster than the 7700k (and still faster than the 8600k) in anything, but gaming. 
https://pcpartpicker.com/list/CmQHvV

You can't really fit a GTX1070 with the current crazy gpu prices in europe into that kind of budget.

I'm running almost the same config except my CPU is a 1700 instead of an 1700x, but even the lower clocked 1700 is enough (with the 1060) for running almost all AAA titles at 1080p on very high to ultra settings at 60fps+. But you should really get the 6GB variant of the 1060, because there are some titles that need more than 3GB of VRAM even at 1080p. If the only thing you plan on doing with your PC is gaming than you can save some money buying a cheaper CPU such as the 1600x.

I'm not familiar with their newer product, but I have a Biostar TH61-ITX from 2012 running as a second PC and it still works find today.

The ASROCK Fatal1ty X370 Gaming-ITX/ac is a nice AM4 ITX board with AC wireless, but it's a little more expensive than the biostar one. But if you're fine with B350 the GIGABYTE GA-AB350N-Gaming WIFI should be cheaper than the ASROCK one.