Any experience with Neewer filters?  Any color shift, loss of contrast/sharpness, Vignetteing, or other issues?

I tried using my Osmo+ outdoors and it's basically unusable in the sunlight without a super high shutter speed, which isn't what I want, so I need filters.  I am on a tight budget though so I wasn't going to pay $30 or more each, so I bought Neewers, they are a fraction of the price, but how is the quality?

I got:
1 CPL
1 ND8/PL
1 ND16/PL
1 ND8
1 ND16
1 ND32
1 Adjustable ND2-ND400

All for $61.98 shipped.

Which one should I use for a sunny day?  How about partial clouds?  Cloudy?  What if I move from outdoors to indoors or vice verse?

I want to shoot at full 4K at 24 FPS with a shutter speed of 50 and ISO 100.

Best is you try out for yourself which filters to use with your camera at which time. For a sunny day, you might consider starting at ND32 and check the frame rate. For partial/cloudy I often use 16. It takes a while to get a "feel" for light, but you will get there soon!

I purchased the newer variable 4/400 filter and wasn't terribly impressed.  https://www.amazon.ca/gp/product/B016W4IAR2

I only bought it because I could get same day shipping, otherwise I'd stick with the more established manufacturers.

Shooting in NA? Use an ND/16 on most sunny days.  Use an ND/8 PL or ND/16 PL when glair is a concern. The ND/16 will become your go to filter for pretty much anything daytime.  I have an ND/32 but almost never use it.  If your in a sunny situation with a lot of light colored surfaces you might need to bust out the ND/32