I think we are trying to make two somewhat different points which are both correct.
Some are arguing that it is possible to use DX lenses on FF and get good, sometimes even great, results. I don't dispute this.
My point is that I don't think you will get the best out of either your DX lenses or your more expensive FF camera by doing this. Just because you can do something does not mean it is the best option. Those who are suggesting otherwise are either unable or unwilling to concede this.
In answer to WPMChan, I have indeed used my DX lenses on my FF Nikon and I got vignetting and poorer corner quality at most focal lengths when compared to my FF glass.
And with respect, I don't think that showing a SOOC JPG of a still life taken at ISO 22800 proves anything. You could easily have got a better result simply by using a much lower ISO and slower shutter speed. Indeed, you could have got a better result with the D5600 at a lower ISO.
If the OP wishes to use his current glass but wants more features eg better and faster AF, faster frame rates, better buffer, better metering etc, then by all means get another camera. In my opinion the best way to do this would be a higher end DX camera such as the D7500, D7200 or even a D500 if he can get his hands on one for a good price.
The D3500 has a very good sensor and if the OP actually wants better image quality, as opposed to features, the simplest and cheapest way to do this would be to get a more professional lens like the Sigma 18-35 f/1.8 or either of the Nikon 16-80 f/2.8-4 or the 16-85 f/3.5-5.6. The latter sits very well on the D3500 and D5600 and can often be found reasonably cheaply second hand.
Vasilis, have you made a decision yet?
Malcolm, it's getting even worse every day :-(.
A few more things about myself:
If I can describe my two-month old photography in a phrase, that would be "(my)aspect photography". I want to reveal secret/hidden/micro/big/misunderstood/covered/… aspects of the world around me (eventually, my emotions) and bring them up-front (for example, a tiny flower accidentally grown behind an urban garbage collection facility). I like my subjects to be minimal/clean. I’ve taken some typically good pictures with my D3500, some nice portraits, lovely coastal scenes (I live close to the sea), not worthy uploading because you’ve all seen thousand fabulous ones of this type. Nice but boring…. But I most often find myself picking my P950 (crappy images in anything but very good light conditions, but that huge reach is invaluable) and capture smallish flowers under huge zoom so that the background collapses severely in often surprising ways, or challenging scenes in general, again under significant zoom. I enjoy such photo-adventures much more. Maybe that’s because of the D3500’s inherent restrictions. I have uploaded a few samples of my P950 shooting (all but one). They are low-res smartphone versions, but you will get the idea. All shot jpg. All but two of them, unprocessed. All shot hand-held even at 2000mm equivalent zoom, under single-shot mode. Very few are decently focused, but I still somehow like them. I was wondering what would happen if I had a chance of a decent body (decent AF, low-light performance, dynamic range and high FPS).
However, my desire to upgrade is partly attributed to my evolution in the field but also to (even more so) my severe Gear Acquisition Syndrome, I guess. Unfortunately, due to recent family problems I am under a very tight budget at the moment. I was very close to buying a mint condition (6K clicks) second-hand D7500. It was selling for 570 Euro (around 640 USD). I offered 470 Euro and was rejected. I then offered 500 Euro and got rejected too. Then I stopped. There are a couple of decent D7200s at 400 Euros available at the moment. Then the whole discussion (triggered by my reading online) came about. Then came you, this wonderful community, trying to shape my attention/desires under more rational thinking about my choices.
Then last night I was emailed an online offer of 999 Euro (probably gray market, but also plenty legit ones of 1299 Euros) for Z50 II, double lens kit (16-50mm & 50-250mm). And I read about the mirrorless marvel, the inherent ability to preview your exposure before the shot, the 11 fps, the 209 focus points, the super-light body (although I am not sure that’s an advantage) the super-duper eye/subject/3D tracking, the -pre -post capture mode just in case you missed the moment….You name it, it’s there. Ironically, utilizing the same sensor as D7500. Made me wonder, shall I just fight my GAS, focus (figuratively) on getting to know my D3500 better, save some money and wait for more mirrorless marvels to come? On the other hand, projecting in the future a few years from now, I don’t like myself having completely missed the brave DSLR era and the opportunity to fall in love and work with one decent representative.
My dilemmas are mostly psychological, I guess, and to a lesser extent rational. Still, quite painful

. My logic says to stick to my gear and wait. My insatiable desire pushes me towards the luxury of shooting with a decent DSLR, enjoy a better AF and overall PQ for 2-3 years more, and then go (if I can, budget-wise) mirrorless.
Still wondering…..