Thank you for your answers @jaymonkey, @Gigamaxx, @Hackintoshron,
I decided to buy a Sapphire NITRO+ RX 5700 XT which I received today.
Everything went pretty smoothly. Here is what I did:
1. Replaced my old Titan X Maxwell with the NITRO+ RX 5700 XT
2. Opencore 0.6.3 with boot flag...