Download the FileZilla Client:

https://filezilla-project.org/

Your upload speed is 2 MB/s, so it will take a very long time to upload a 1 GB video.

                      App. 1 GB video takes roughly 3 hours to upload. Is not that toooo long?

If your videos are 1 GB each, you're doing it wrong.  Compress, compress, compress.

                    I also think that my videos are too bulky. How to set daVinci Resolve export to get smaller file? Thanks

Compress 4K with H.264 at 50 - 100 Mbps.

...but the video will not be 10 bits then.

10 bits schmend bits.  At the commissions we're paid these days, buyers can have 8-bit videos.

H.265/HEVC is a more efficient codec than H.264 and provides greater compression efficiency and better video quality.

The downside is H.265 is more taxing on low-horsepower computers/devices and isn't as widely supported as H.264 but Davicvi Resolve supports it    Some stock agencies will accept H.265 clips (Pond5 and ?) but others don't (Adobe and ?).

https://techreviewadvisor.com/h-264-vs-h-265-hevc/

H.265/HEVC is a more efficient codec than H.264 and provides greater compression efficiency and better video quality.

The downside is H.265 is more taxing on low-horsepower computers/devices and isn't as widely supported as H.264 but Davicvi Resolve supports it    Some stock agencies will accept H.265 clips (Pond5 and ?) but others don't (Adobe and ?).

https://techreviewadvisor.com/h-264-vs-h-265-hevc/

Shutterstock does not accept this codec. Other stocks accept, but only if it is the original video from the camera. I think buyers will not buy H.265 either, it is an inconvenient codec.

Great news, SS just approved my videos: shutterstock.com/g/OttoPles/video
 Those bulky files were not approved, because they are over one minute /1,05/
                      The lenght is the key factor as well. My 26 second clip is: MOV, 473MB, total bitrate 147000Kb/s
                       Do you think I can make it smaller? If I set in daVinci Resolve  export restriction to 100 000 kb/s the size get app 10% smaller. That the way is suppose to be. Thanks a lot.

 Thanks Stoker2014
                       now things are much clearer to me. I tried 60Mb/s bitrate and on my 27inch monitor i did not see any differnce.
                       The question is whether  majority of potential buyers look for best quality, or most of them are OK with like 60Mb/s bitrate.
                   I bought Lumix S5II for video, and noticed, that I can just set frame rate to 25fps, or 50fps/Europe/. No 30fps or 60fps
                   Mostly I shoot 50fps.
                  I learned just basics on daVinci, since almost all videos I slow down from 50fps to 25fps.
                   Also I set my camera: 420/10-L, because I have free version of daVinci. Thanks again

                        The question is whether  majority of potential buyers look for best quality, or most of them are OK with like 60Mb/s bitrate.

You started the topic by saying that you have a bad internet connection and you need a small file size.
In any case, I don't think you'll see a difference in sales between bitrates of 60, 100 and 150.
On many stocks, the buyer doesn't see the bitrate in the video description.

Ideally, of course, you should shoot in 10 bits (which your camera does) and render the video in the prores 422 codec. Then the buyer will get 10 bits. Any other options are the final video in 8 bits. Usually, for the H.264 codec, many set the bitrate to 100 or 150, I don't see any difference.

                   I bought Lumix S5II for video, and noticed, that I can just set frame rate to 25fps, or 50fps/Europe/. No 30fps or 60fps

Your camera is better suited for photos than for video. Better change it to Panasonic GH7 or GH6.

Read the manual. You need to change the system frequency to 29.97, i.e. to NTSC, if necessary, change the region to the USA. Then you will no longer have 25 and 50 frames, but everything will be in NTSC (i.e. 29.97 and 59.94).

                  I learned just basics on daVinci, since almost all videos I slow down from 50fps to 25fps.
                   Also I set my camera: 420/10-L, because I have free version of daVinci. Thanks again

Perhaps your camera can also shoot in 59.94 and immediately convert the video to 29.97. Many Panasonics can do this. But in any case, you can shoot in 59.94 and then convert to 29.97 on your PC. You can also shoot in 8 bits and 10 bits. If you render the video in H.264 and do not do color correction, you can shoot initially in 8 bits.

optical cable for faster internet I will get within 24 months.
                           

 
 

Aim for 15 seconds clips

Aim for 15 seconds clips

From advice of others, 15 to 30 seconds at most. A longer clip can be made into more shorter clips to offer.