One of the easy way would be
[dual mini 6pin -> single 8pin] + [Single 8pin (female) -> dual 6+2 pin]
I am using this bridge method to power my 1080Ti. If this cable combination can power a 250W graphic card, then it should be very safe to power your 215W 1080 FTW.
The idea of using a bridge in between is to balance the demand. So, no mater how the card designed or programmed, the power draw can still virtually evenly distributed across the two mini 6pin to avoid the self shutdown protection. And this is how my 1080Ti draw the power when under stress.
View attachment 750017
As you can see, despite my card has 6+8pin config, both mini 6pins always work together to share the load. So that the card won't over stress one of the mini 6pin, but under utilise the other one.
One of the common wrong assumption is that your card will draw 75W from the slot. Therefore, since your card is 215W TDP, each 8 pin will only draw 70W (70+70+75 = 215). So that, you can connect each of them to a mini 6pin individually, and still within the mini 6pin official limit.
From the above chart, you can see that there is no guarantee that your card will pull 75W from the slot (slot 2, in my case), not even when running Furmark. Especially your card has 2x 8pin, there is no point to stress the slot to 100% and only utilise 47% on each 8pin input.
I expect your card's power draw's pattern will be similar to my card. Only pull 40-50W from the slot when under stress, and the rest are from the 8pins. So, there is a very high chance that even your card's TDP is below 225W, but it will still pull more than 75W from the mini 6pins.
[Update: I found this
review which shows that your card only can pull about 30W from the PCIe slot. And in extreme case (Furmark), it can pull up to 232W in total. Therefore, it means each 8pin can pull up to around 100W]
Another common wrong assumption is that because your card has dual 8pin, therefore, it can draw up to 375W from the PSU. I am 100% sure it won't, and it can't. No matter how many 8pin inputs are there, it's just the "power available", nothing to do with the "real power consumption". At the end, it's just an air cooled 1080. Manufacture said it's a 215W card, so, it's power draw should not go above this number without user OC or intentionally running power virus (e.g. Furmark).
IMO, that dual 8pin is just a marketing strategy. It make the consumer believe it's a powerful card because has dual 8pin (compare to a normal 1080 which only has single 8pin). In fact, my 1080Ti only has 6+8, but it's a 250W card, and it's definitely more powerful then the 1080FTW. That dual 8pin design is more likely for extreme user OC (which you cannot do in macOS), and to relief the slot (no need to pull all 75W from it). Therefore, you shouldn't need extra PSU or Pixlas mod. Nothing stop you to do that, but just not required.
[Again, from the same
review, your card only draw ~200W in heavy gaming. For your usage, I think it can hardly touch 200W]
The 3rd common wrong concept is that the card will automatically balance the load between the two 8pin. Therefore, you can connect each 8pin to each mini 6pin. Yes, it may happen, but from what I see, it won't be that perfect most of the time. The card may still programmed to heavily rely on one 8pin, and very under utilise the other one. Therefore, you better to do the balance by yourself to avoid over stress one of the mini 6pin (which can trigger the auto shutdown).
