Micro SD failure/lack of storage space

Given TF cards date back to 2004ish (???) I'd suggest you switch to the more current, actively developed and supported microSD cards:
https://simple.wikipedia.org/wiki/MicroSD
Fifteen year old storage media may still work but it's not surprising they don't work reliably in a more modern mobile device.

 

What is the storage capacity size of the microSD cards you've been using?
Your phone model is a relatively low-spec device with limited capacity, if the cards you've been using are larger than 128GB (the recommended maximum for your model) that might explain why there are issues with reliable storage.
https://www.pdevice.com/product/wieppo-s5-price-specs
As for setting up peripheral devices like a FitPro, you do need to be very, very judicious about apps and such that you need to add. With only 1GB of RAM and 8GB of internal storage, your phone just doesn't have a lot of spare system resources.

 

Do you lose all data on the cards, or just some (e.g. older files)? Where are you buying the cards from?

Existing files starting to disappear or being corrupted could be a sign of a "fake" card. These are small cards reprogrammed to tell the device they are larger cards then sold at a higher price (though often still "cheap" for the size). Since the phone thinks it has a large card in it it keeps writing data even when the card is really full, which leads to existing data being corrupted. If you buy cards from places like eBay, certain Chinese online markets, fly-by-night Amazon marketplace sellers there is a very high risk of getting one of these.

If the entire card fails irrecoverably (e.g. a master boot record failure, which most recovery utilities won't be able to get anything back from) all I can do is quote the dictum of Auric Goldfinger (Bond villain): once is happenstance, twice is coincidence, three times... I'm sure you get the picture: if your cards keep failing catastrophically like that, particularly if they come from different suppliers, there is something badly wrong, and the common factor is your phone.

 

There are plenty of really usable phones, from big name manufacturers, that are not in that in that pricier, flagship class of phones:
https://thewirecutter.com/reviews/best-budget-android-phone/