APS-C or Full Frame: yes, a Full Frame camera is nice, but in comparison to a smartphone, an APS-C DSLR with a lens still will blow the phone out of the water. It all comes down to the optics. I am a Canon guy. I have been using the Canon Rebel Series APS-C models for many years until I bought a 5D Mark II when its price came down (the 5D Mark II still is an awesome camera today although it is way outdated). The step up to full frame was very nice, but the photos were not bad before that either. This year I donated myself a Canon EOS RP (more than enough for me). Full Frame photography has become affordable. But any DSLR will give you a lot, no matter if APS-C or Full Frame. Learning and using the exposure triangle can be a lot of fun, regardless of the sensor size (even on an iPhone with apps like Halide).
@zkarj - I can only return what you wrote about my post: well put! 
I have to be honest: I do not consider myself a good photographer. I have been impressed with several shots on my iPhone - I am not sure, if I could have achieved the same with my skills on my DSLR. Sometimes the processing on the iPhone is just great. Thank you for mentioning DxO PhotoLab - something to check out eventually!