I used Nikon and Canon SLRs and DSLRs for over 40+ years, plus some Leica Rangefinder. I moved over to mirrorless in Jan, 2017 for all its advantages that no DSLR can physically have.
I went with Sony mirrorless. I have owned Sony APS-C A6500; Sony fullframe A7RII, A7RIII, A7III, A9; and currently own Sony fullframe A7RIV, A1, A7SIII. I currently own 13 native E-mount lenses covering 10mm to 600mm from Sony, Sigma, Tamron.
For me personally, Sony stills has the most complete and proven fullframe mirrorless system. And the E-mount is still the most widely produced and used mirrorless lens mount in the world with currently over 200+ native E-mount lenses and constantly growing in number from Sony, Sigma, Tamron, Samyang/Rokinon, Tokina, Viltrox, Zeiss, Meike and more brands.
https://en.wikipedia.org/wiki/List_of_Sony_E-mount_lenseshttps://en.wikipedia.org/wiki/List_of_third-party_Sony_E-mount_lensesI have still held on to one of my fav SLR, my old Canon F1 SLR, like I used when I covered the War in Nicaragua, between Contras and Sandinistas. I still use it with some Canon SLR lenses I kept, when I teach my 35mm B&W film course at a state university, where I also teach Photojournalism and Digital Photography courses. I have an old first generation Nikon D1 DSLR but it isn't any longer in working condition.
Others may prefer Nikon, Canon, Fuji, Panasonic, Leica or OM mirrorless and that is fine. Use the brand you like and be happy
Cheers and best to you.
I used Nikon and Canon SLRs and DSLRs for over 40+... (