The Sony A6x00 cameras only have one wheel on the top rear and one dial at the rear of the camera, which is too few, in my opinion. The full frame A7 series have a dial at the rear and a dedicated EV comp whell and a main and sub command dial, actotal of four, which gives a very satisfactory experience. The wheels and dials are programmable. The number of wheels available make the Sony A7 full frame series much better cameras than the A6x00 cameras. Thus, if you go for Sony, you should go full frame. I use Sony full frame A7 and A7 II, and I'm very happy with my choice. The presense of more wheels and buttons make all the difference to the APS-C cameras. The Sony APS-C bodies are not much smaller than the full frame A7 series of cameras, with the biggest difference being the film SLR like lump for the viewfinder.
There are several programmable buttons and some lenses have a focus hold button that can be programmed for other purposes. The Fn button gives access to a quick menu that is also customizable.
In general the Sony user interface is customizable and computer oriented. If you're old school, Fujifilm may be a better choice with its very analogue user interface, somewhat resembling a manual focus film SLR.
The downside to Fujifilm is the absense of IBIS, in body stabilization. The Sony A6500 as well as A7II, A7RII have got IBIS.
Native AF lenses
The selection of native lenses for full frame range from 16mm to 300mm, with 16-35mm (10mm rectilinear MF wide angle) to a 70-300mm telezoom at each end. If you require supertele lenses, you need to use adapted lenses. (Since you have Nikon, stay with Nikon for supertele lenses.) The lens selection for full frame is better than for APS-C, and the lens programme for full frame is much better thought out. Sony's lenses range from the 16-35/4 to the 70-300mm tele zoom and Sony, Zeiss and Samyang AF primes from 14/2.8 to 90/2.8 macro.
Native MF lenses
If you're into manual focus, Zeiss and Voigtländer offer some nice full frame option with electronic integration, Voigtländer 10/5.6, 12/5.6, 15/4.5 (all three are rectilinear WAs) and Zeiss 21/2.8, 35/2, 50/2, 85/2.4. Cosina has announced the Voigtländer 65mm f/2 Macro Apo Lanthar as well. The MF lenses come with old school mechanical aperture and focus rings, and EXIF as well as aperture settings are communicated, much like chipped Nikon MF lenses.
Adapted lenses
Canon EF and Sony A mount (DSLR) lenses can be adapted to Sony E mount with full AF, EXIF and exposure automation. Nikon lenses can be used with mechanical adapters. I advice against trying any smart adapters for Nikon lenses at this point, due to reports of these cheap (Chinese) adapters destroying Sony cameras. Only Metabones IV T for Canon EF lenses and Sigma MC-11 for Canon mount lenses should be used, if a smart adapter is required. For Fujifilm there are no smart adapters.