>>280689872
I'm pretty sure that a GPU upgrade to a 2070s or similar shouldn't cause any issues unless your current power supply cannot handle it. 
You're right that a CPU upgrade is a pain in the ass though - Check what is the most recent series of CPUs that your mobo/ chipset is compatible with. You may be able to buy a used CPU from that series which is better than your current one and should run on your current system. That said, a modern new CPU can do wonders for system performance