This is were getting a old dual xeon build could be useful. A Intel Xeon E5-2670 2.6 GHz 8-Core Processor - 20 MB - LGA2011 Socket - OEM is like £45 each. Cheap Intel Xeon E5-2650L v2 are around £90 or get lucky with the Intel Xeon E5-2660 v2 (I can see some selling for £78-82). So a dual motherboard if you can find one, see
https://www.asus.com/uk/Motherboards/Z9PED8_WS/ see £129.98. Intel Xeon E5-2670 is 16 cores and 32 threads. Intel Xeon E5-2650L v2 is 20 cores and 40 threads.
Then get some RAM, PC3-10600R ECC Registered will set you back £30 per 32GB.
So,
2 x Intel Xeon E5-2660 v2 £180 (20 cores and 40 threads) (3190 points each CPU cpu-z
https://valid.x86.fr/bench/j8ivxm/16 or 6280 point ideal or close to the Intel Core i9-7920X)
Z9PED8_WS £130
PC3-10600R ECC Registered 64GB £60
Total = £370
Sure there is more you will need to buy but this is the same as one 3800x.
So with this you are hoping to get as close to cpu-z 6000 multi-core points as you can. Not sure what you will get overclocking wise but you can try at least. Remember this build is just for video encoding, not for games or going on the internet.
Overclocking which you will need,
https://www.youtube.com/watch?v=xt62JyJo1iU
Or just call overclockers and ask them for advice.