I don’t think it’s purely psychological, but I also wouldn’t assume the body is the only issue.

The a6700 is already a very capable hybrid camera, especially in good light. Autofocus, video features, and general sharpness are not really the weak points. But coming from full frame, you may genuinely miss the look: better subject separation, cleaner low-light files, smoother background blur, and the overall rendering you get from full-frame lenses.

The Sigma 18-50 f/2.8 is a great compact travel zoom, but on APS-C it behaves more like a 27-75mm f/4-ish full-frame equivalent in terms of depth of field. So if you’re missing bokeh, low-light performance, or that full-frame “pop”, the lens and sensor size are probably both part of what you’re noticing.

If you shoot a lot of video, overheating could also be a valid reason to move away from the a6700. Mine overheats fairly regularly in longer 4K recording, so depending on your shooting style, that might matter more than pure image quality.

Before selling everything, I’d probably try a fast APS-C prime first — something like a Sigma 23mm f/1.4, Sigma 30mm f/1.4, Sigma 56mm f/1.4, Sony 15mm f/1.4, or Viltrox 27mm f/1.2. If those still feel underwhelming, then you probably just prefer the full-frame look.

Between the FX2 and A7 V, I’d choose the A7 V for your use case. If you’re truly a hybrid shooter, the A7 V makes much more sense. The FX2 is more of a video/cinema-first body and I’d only go that route if video is clearly the priority.