3060 12GB are the best things you can buy right now. They are cheap, have a ton of memory--which seems to be the issue w/ image generation--and you can fit four of them into the cheapest motherboards.
3060ti 8GB, 3090 24GB, and 4000 series all have performance benefits, but for now this one is off the charts.
Another one to consider is the A4000 16gb. I recently bought an ex-miner card for ~$500 usd . They are around a 3070 with a decent amount of memory for training scenarios, and are single slot cards. I believe there are a lot of these workstation ex miner cards which are pretty heavily discounted.
Combine this with a second hand X99 / 2011-v3 platform like the Dell Precision T7910 dual socket Xeons and you can have a pretty decent homelab for ML workloads. The Dell can come with a 1300 watt PSU and can fit 4 of those cards comfortably (5 with reduced PCIe lanes on one) since they are 150W each.
For image inference 12GB is ok for now (but you may not be able to use all future models given that T5 language model is becoming popular), for training I'd consider 24GB the bare minimum.
3060ti 8GB, 3090 24GB, and 4000 series all have performance benefits, but for now this one is off the charts.