Hi and welcome to the forum:
On the contrary to your belief it is a burden, our members flock to this kind of question like moths to a flame.
The P520 is a 12 YO bridge camera with a sensor about the size of one from a cell phone, so any of the R-series will seem like a massive increase in resolution!
Having shot both subjects, I would suggest that landscape tends to demand the detail and clarity of a full-frame camera, and often moderately wide-angle lenses to telephoto, whereas aviation photography will demand normal to longer focal length lenses. Focus is going to be a big benefit for the latter genre. In that respect, I much prefer the superior focus and face/eye tracking of the R6, along with its fabulous 24MP FF sensor that is great for clean ISO and dynamic range: again great for landscapes. It has a top end processor, great buffer and you can add a battery grip to it as well. It is also weather sealed. Finally, it takes two SD cards to security of backing up your files as you shoot.
The most development range has been with the full-frame R-series and I would recommend the R6II and the RF 24-240.
You can browse the current selection of bodies from Canon's Refurbished site, which offers over-stock but new, open box but unused, demo or show units 'good as new' with a Canon warranty, and some savings:
https://www.usa.canon.com/shop/p/refurbished-eos-r6-mark-ii-body (you can request to be notified when available)
This is a prosumer body - superior build, weather sealing, IBIS, top of the line focus and Face-tracking, high end sensor with excelling ISO performance, and top-end processor. It's controls are highly customizable and it will perform well with future high-end RF lenses.
Pair that with the excellent RF 24-240 IS USM super-zoom lens and you will have a serious kit that can be expanded later with RF glass.
https://www.usa.canon.com/shop/p/refurbished-rf24-240mm-f4-6-3-is-usm
Here are some sample images with that lens on R5 and R6 bodies. For most of your photography, this would be a single lens that does a lot, very well.
One Test of a RF 24-240 Super Zoom Lens - Does it ... - Canon Community
Moody Morning the the R6 & RF 24-240 - Canon Community
Trying out the RF 24-240 on Wildlife with the R5 - Page 2 - Canon Community
I love this lens and it punches way above its price point.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
