Do you think it would be worth it to buy a new motherboard and a new cpu? Or should I just look at buying a new machine completely?

These are your options:

• 6 th Generation Intel Core i3
• 6 th Generation Intel Core i5/i5k
• 6 th Generation Intel Core i7/i7K
• this is a somewhat older machine I would honestly not bother upgrading the CPU because the price versus performance increase does not make a lot of sense. 

I would get a complete new machine when there's a sale going on, as often you can get good deals.

I got a brand new R10 with an RTX 3080 last year for $2,000 USD, as it was on sale for -$1,000. It was regular $3,000.

Considering the pricing on the RTX 3080, that was a good deal.