The D3500 is pretty much an entry-level DSLR. It is a fine camera, but like all DSLRs it is bigger and heavier than it needs to be. You already owned a mirrorless camera before, and unless you had a terrible experience with it I would stay with a mirrorless system. They generally perform better and are lighter than their DSLR counterparts.

If you are happy with your A5000 you just might look at a different lens for it. Sony has added many APS-C lenses since you got the A5000. The 18-135mm lens, for instance, is a fine lens; perfect for travel. It has image stabilization which will help you get sharper images. It goes for about $500.

I would look at that Sony a6000 series. Or at a micro 4/3 system--Olympus, Fuji, Panasonic, etc. All these systems are travel-friendly and give you great image qualitiy. This would set you back a little more than $500. But given your travel activities this might be an investment that is going to make you happy.