Short version: Safe bet is Sapphire RX 580 Pulse with PCIe dual mini 6-pin to PCIe 8-pin.
Long version:
If you really need more graphics power without a mod, there is a cable combination you can use with some Vega 56 models. The card would be a non overclocked version, probably equipped with 8-pin and 6-pin PCIe power sockets, and it would need to have a dip switch for switching it to power saving bios.
This is what I have used for over 8 months already.
Powercolor Vega 56 Red Dragon, switched to powersave bios on, and PCIe dual mini 6-pin to PCIe 8-pin AND 2 x SATA power to PCIe 6-pin.
You have to find out if there is the switch available. Some cards do have it, some don't. There might be two dip switches and you have to know which one is the right one. There might be labels with switches, or maybe there aren't.
Performance is good even with low bios mode, and it's faster than RX 580. Please note that I do not use any SATA devices with this combination, I have a NVMe SSD in PCIe slot.
If you don't do some 3D GPU rendering or maybe heavy video compression, I think RX 580 would be a good option.
ps. Now I have couple of Radeon VIIs waiting for suitable clips to do Pixlas mod. You can run the VII with above mentioned power feed from motherboard and dual sata, but it would not be safe to load GPU that way. It would eventually burn something, or at least shut down the machine.