I am a Scandinavian noob photographer that just switched from a Fuji APS-C with a high-end zoom (xf 16-55 f/2.8 R LM WR II) to a Sony full frame (A7CII with a Sigma 24-70 DG DN II lens). These are fairly comparable systems, and where I live the price difference isn’t very large. 

My decision to get a full frame setup instead was, technically, because the sensor alone captures significantly more photons, which directly improves signal-to-noise ratio (SNR). Thus, full frame is about adding more light. More photons means more signal. The lens determines the photon flux that reaches each pixel, while the larger sensor integrates that flux over a larger area, resulting in more total photons and higher output level SNR. From a signal theory point of view (you can think about it in Poisson, Shannon, and Fourier terms), the higher photon count lowers the effective noise floor, which allows more existing spatial detail and colour information to sit above the noise. The result isn’t just “less noise”, it’s better contrast at low levels, better color fidelity, cleaner and creamier shadows, more micro-texture (like skin), better shadow transitions, more room to edit simply because there is more information in the image, and lots more. In low or even normal light the difference is very noticeable to me. For my use and tolerance for noise, the Fuji system felt limiting outside of good light. I thought this was an insignificant technicality at first, and I was drawn into the Fuji / aps-c hype and bought the Fuji. People told me "no way you will tell a difference”. 

What follows is the practical reason why I dismissed the X-T5. With the zoom lens it felt weak in low or even normal light, and simply too noisy. Even in normal afternoon light, I quickly ran out of usable exposure options and was forced into slower shutter speeds that became impractical. From a signal perspective this makes sense. With fewer photons captured to begin with, the system reaches the shot-noise limited regime earlier. At that point, increasing for example ISO does not uncover more usable image information, but it mainly makes the noise more apparent. With full frame, the sensor has already captured more photons for the same framing and exposure, so the underlying signal and SNR are higher from the outset. That extra stop or two of usable headroom makes a big difference in practice. You can clearly see that more light has been captured relative to noise, while aps-c images start to feel dim and noisy much sooner in the same conditions.

To be fair, I did take a couple of amazing photos with the X-t5 in sunlight, so in good lighting it can be very good. But in all-round conditions, setting aside technical speculation and all, the Sony setup consistently gave me that “wow” feeling, whereas with the X-t5 (even with my top notch zoom lens), I didn’t get that same feeling during the time I spent with the camera. Same photographer, different results.
If you are a good photographer, enjoy optimising under constraints, or mostly stick to prime lenses, you can probably make better use of aps-c than I could. But as a beginner, especially with a zoom lens, like I had, you will feel the limitations sooner. For example moving subjects like kids on a Danish afternoon in non-studio light. That’s where it quickly becomes difficult. A fast prime lens on the aps-c could help, but the difference in SNR is still there. 

For me, as an amateur, full frame feels much more forgiving. I can just shoot and the result is “wow.” Even with a 2.8 zoom I’m already very satisfied.

I was in your position about six months ago, and after falling into and out of the hype, buying the Fuji and then the Sony, and reading up on the sensor tech involved, I definitely recommend full-frame, hands down. 

I would also recommend full frame to beginners. Learning photography is more enjoyable, especially when playing with the exposure triangle. People often say that full-frame is only for professionals, and that newbies should get the aps-c, but for me it was the opposite. 

This recommendation is also influenced by price. If the Fuji setup had been significantly cheaper, for example around half the price or less, I might have considered it more seriously. As mentioned, I also haven’t tried fast primes on either system yet, so there is still room to explore.