New AMD or Nvidia GPU for more support?
Time for a GPU upgrade, I know not the best of time to do that due to the ongoing shortage, but with no end in sight, I suppose I just have to bite the bullet and break down and get a decent card. I was using a R9 280x which has served me well until last week when it started flickering. After further testing on a different PC, it turns out the GPU is dead/dying.

For hashcat use on Linux/Ubuntu 20.04, which more recent cards are more widely supported? Nvidia or AMD?

Thinking of either of these 2 cards:

RTX 3070
RX 6700 XT

Any known issues with any brands of more recent cards in Ubuntu 20.04?

I ask this because I couldn't get the R9 280x to work properly on Ubuntu 20.04 and Windows 10 (opencl memory errors), most probably because of drivers and the fact that the card is almost 8 or 9 years old (in terms of driver support).
If your budget is around this level, AMD's generous helping of VRAM here means that you're getting much better performance in higher-spec games than Nvidia's equivalent GTX 1650 can offer. I also recommend you AMD is best for your Laptop. I also have AMD for my hp laptop rental. I buy AMD at very best prices and its work very well now.
For Hashcat use go with NVIDIA, AMD means calling for troubles.