The burning question is - are you going North to South or South to North. It makes a difference because with South to North if you have the solar panel fastened to your backpack you will be getting at least some direct rays onto the solar panel. In the other direction, for the most part you will only be getting incident light, not direct ways, which means much reduced output if any at all.
Anker makes a couple of nice portable panels in the 15-21 watt range. But be careful - they are using solar panel output to make that claim, and that is not what will be available at the USB port(s). Look at the fine print for the total amps available at the USB port(s), multiply that by 5, and you will have a much better idea of what you can expect for output.
Personally, I would stick with the solar to power bank, power bank to charger route. The reason being that as you move about, even in direct bright sunlight, the output of the solar panel will vary based on its orientation at that moment. I haven't read any reviews that discuss how a given charger will work with a constantly varying current input. I think using a power bank would tend to negate that kind of problem, although I have to admit I don't know if any power bank, since it is in effect a charger, will handle varying input current either. I would recommend contacting the manufacturer of whatever charger/power bank looks good to you and ask them that question. It might save you from some severe disappointment down the road. I do know that my Maxboost 15000mAh power bank didn't seem to have any problem handling varying input current when I was testing my solar setup.
Good luck and enjoy your trip. Let us know how what you select for your journey, and how well it performed for you.
PS: I would suggest getting a small AA or AAA light as a backup in case of prolonged lack of sunlight, or if something happens to your normal setup. I know there will be stores along the way, so you should be able to pick up more lithium primary AA or AAA cells if needed. I say lithium primary because they are lighter in weight than alkalines, and last longer in use. A Streamlight Microstream or a Lumintop Tool or its Manker equivalents would be ideal, and not very expensive. A Nitecore LA10 CRI would also work, and would make a great tent/camp light as well. When it is fully retracted it puts out a nice soft flood of light that should work very well for any night time wandering.