With both your older iphones, you have batteries inside of around 1800mAh.
I use a rule of thumb to roughly guess how much power I'll need for a particular trip. For example, I use an estimate of efficiency of 80% of the powerbank capacity (in mAh) to allow for losses thru conversion/heat/transfer/etc). So for your example, the smaller (lighter charger and separate cell battery) options might only charge 1.5 times per iphone - 3600mAh x 0.8 / 1800 = 1.6. For a 10000mAh powerbank (10000 x 0.8 / 1800) you might get around 4.5 charges from a flat iphone.
This will give you a rough idea of how many charges you can get with your powerbank. So depending on how you use your phone and how many times you expect it to be flat, you can work out a rough guide as to how big (in capacity) a powerbank you'd need.
I nerd out a bit more and test how much my powerbanks output via some inline ammeters, and have found different brands have higher efficiencies up to 90%, but if you don't have access to that sort of thing, the rule of thumb works out as a general guide.
For powerbanks I have had experience with Anker and quite a few other brands. For best bang for buck, the Xiaomi powerbanks offered good alternatives to the Anker powerbanks.
For lighter powerbanks, go with the ones that have a flat lithium ion internal cell, like the Anker PowerCore Slim 10000 for example, or the Xiaomi 10000mAh Mi Power Bank 3. If you compare these "slim" versions side by side with the older fatter 10000mAh versions, the fatter versions are inevitably heavier, as they contain a series of cylindrical 18650 lithium batteries inside.
There's some previous discussion on the Xiaomi powerbanks here
viewtopic.php?f=21&t=19706&start=60 with photos of the "slim" versions vs the "fat" versions.
For small solar panels, have a look at this thread -
viewtopic.php?f=21&t=24317&start=60Avoid tiny solar panels (eg the ones the size of a phone or a paperback book). They are too small to charge up a powerbank adequately in a day. Avoid powerbanks with "built-in" solar panels for the same reason, and the fact that the powerbank will get hot sitting in the sun, and lithum batteries don't charge well over 45°C. Get a panel that can restart, or if failing that, a powerbank that can accept charging restarts (when the panel is obscured by clouds or shade).
I've not used the Klarus chargers you mentioned, as I have too many chargers already at the moment, but note that some chargers do not work well with solar panels. In fact, if I was going this route, I usually avoid using chargers with a built-in LED display with my solar panels. The main reason is that the display takes some of the power from the solar panel to power the LCD screen. I would prefer using a charger with just a small LED light to indicate charge state when using with a solar panel.
If you are bike packing/ cycle touring, you can probably carry a larger/heavier panel. I have had success in using larger 21W folding panels, like the Anker, Goal Zero or Blitzwolf trifold panels which can weigh between 500g to 1kg. Larger panels usually = higher charge current = quicker charging.
With the small lightweight panels mentioned in this thread,
viewtopic.php?f=21&t=24317&start=60 I would get a charging current of between 0.6A to 1A in optimum sunlight in real life use.
So with that information, assuming a perfectly sunny day, where you are able to angle the panel at optimally the sun, you can roughly calculate how much power you can generate from the panel. Eg. Assuming you can achieve 0.75A (750mA) on average, with minimum losses, a 10000mAh powerbank would take at least (10000 / 750 = 13.33) 13.33 hours to fully charge. This is a very rough estimate, but should give you an idea of what would suit your needs.