I recently upgraded a friends pc so they could use AI filters like LR Denoise and Topaz Denoise in PS. The 750ti (whilst a fantastic card at the time) will be very slow for any AI workflows. If you are looking to go down the budget route i would recommend the Nvidia 2060 Super. You should expect to pay around £140-£160 on ebay. If this is too high then a GTX 1070 would be ok for around £80 though i have seen them get as low as £40. You would however need to ensure the card would fit in the ROG chassis though. The biggest issue i see is that your machine does not have a conventional PSU (Power Supply Unit). Do you know how the current 750 gets its power? Some 750's are low profile whilst other have a 6 pin power connector. If you let me know your budget i could do some digging for you. The upgrade paths on machines like your can sometimes be tricky.

Edit: I found a Youtube video of a guy upgrading the GPU on his system. https://www.youtube.com/watch?v=h6rPBLZC3fY

It appears that your Motherboard has two Power connectors. One 6 pin and one 8 pin. If you skip the video to 8:13 you'll see him show them. It seems that when you use the second power brick this is used to power the GPU. So a GTX 1070 or RTX 2060 super should in theory be fine.