The liquid crystal display meters (LCD) cost a bit more, the light emitting diode (LED) meters are cheaper but very hard to see in sunlight.
https://www.ebay.com/sch/i.html?_from=R40&_nkw=usb+current+meter&_sacat=0&_sop=15&rt=nc&LH_BIN=1

I got one of these recently, works great and is readable in sunlight.
https://www.ebay.com/itm/UNI-T-UT658...C/232596812253

Stuff shipped from asia can take a bit over a month to get here, and if the trade war gets worse it might not get here at all.

I have not used the app you are using. Can you use that in airplane mode with the screen off? Or does it need to stay connected to internet? If so, about half of my tours are remote enough that I have no cell coverage, so if you need cell coverage that could be a problem. The screen takes a lot of power, if you can leave that off most of the time, that will help.

Bring a big power bank that can power your phone for a couple days if you have overcast skies.

Do you know where you are going? This can help estimate how much of the time you can expect clear skies. Enter your proposed location into it.
https://weatherspark.com/y/757/Avera...tes-Year-Round

This is several years old, thus a lot of the hardware described in here is no longer made, but overall it is a good discussion.
https://www.crazyguyonabike.com/doc/?doc_id=9258