Things like sampling for collage have a different historical record of successful defense under the same laws from things like sampling for music. (Tends to be viewed more favorably for some reason, yet still winds up in court if someone objects to that use of what they did.)
But collage isn't exactly the concern here. Given you said 'many photos', I'm not sure what is, exactly, but just in case: Looking at a bunch of different photos to figure out how a horse works? You aren't copying a photo, you are accurately establishing how a horse works. Getting the details of something you want to paint correct doesn't (generally) mean you are copying the photo. If you look at 100 photos to figure out how to convey subsurface scattering, or a particular setup of lighting like light from below, and then use those on your piece, you aren't copying - you learned something.
I am, in fact, someone who likes to paint directly from (a single) photograph that someone else made. I don't really see the difference between feeling the urge to build some additional interpretations on a lovely photo someone else has taken and feeling the urge to capture a photo of something yourself and build on it later, except that the first gives you a link with someone else, and opens worlds you can't reach.
If you also want to use photographs like this: find photographs made available for your use by the photographer. The reference library here is. Public domain images are, and some of the images released into the public domain due to being government photographs are quite beautiful. Creative commons licenses are often used by photographers who'd love for you to use their work if you attribute them properly and/or also make your work usable under the same terms. You can often buy the privilege to use a copyrighted photograph in a painting from a photographer and they will enjoy the chance to collaborate.
At the end of the day keep in mind there's a fellow artist on the other side of that camera. Honor them without doing them harm or going against their legal rights and wishes.
(And understand that there might be some obscure legal clause you missed that reverts copyright on a work-for-hire to the original creator after 35 years and you might have to cease copying derivative works at that time; or that perhaps the license you had permission to use under was one that could be withdrawn at any time and they might change their mind.)
Disclaimer: I am a programmer and the open source movement has gone mainstream enough to be rather indispensable to my usual creative process. An amount of the ethos naturally carries over to my hobbies. Trying to figure out how the different open source licenses you are using interact can be ... fun ...