How is it possible that my high resoltion photo's in my library for clients is 300 dpi and color profile sRGB IEC61966-2.1, and when my clients download these photos they suddenly are 72 dpi and beeldschermP3??????????
First of all, it isn't DPI but rather ppi (pixels per inch). You cannot change the quality of an image by changing that number. If an image is 6000 x 4000 pixels then that is its resolution regardless of what that ppi setting is. Are your master images in Lightroom Classic on your desktop, or are they in Lightroom, and therefore in the cloud? What is the size in pixels of the images that your clients are able to download? That is the key number to be concerned with.
When the client downloads the images, it is effectively an export, i.e. applying your edits to the original file in order to create a jpeg. Jpeg is a "lossy compressed" format, meaning that some degree of compression is always applied, so invariably the result is a smaller file in terms of the amount of disk space it uses. The amount of compression will be determined by the quality setting, the lower the quality setting the more compression is applied and thus the smaller the file will be. I'm not sure, but I think the quality setting used by the download process will be between 80 and 90, which is plenty good enough (you would probably find it difficult to see any difference in image quality of a Jpeg exported at 100% compared with the Jpeg exported at 80%). The important thing therefore is to check that the resolution (pixel dimensions) of the downloaded files is the same as the original files in the cloud.....but note that if the images in the cloud were synced from Lightroom Classic, they will in fact only be smart previews, 2560px on the long edge, and not the originals at the full resolution.
Ignore the 72dpi flag in the downloaded files, it has no impact whatsoever on the actual downloaded file.
I can't say whether the P3 profile is an issue or not.
Are you uploading and presenting the images to your client FROM Lightroom Classic? If you are then they are viewing smart previews, NOT full-sized images. That would account for the reduction in size. Again, if that is the case, presenting images to your client that way would be a good way for them to look at the images but it wouldn't be the best way to deliver them if you want the client to receive full-sized images.