Photos greyed out in iPhone and iCloud, 'unable to download' error message

Hi community. I've been an iPhone user since 2013. Through the years and upgrades, I've noticed that many of my photos get greyed out on my iPhone and I cannot view them even when I click on them. When I went to investigate in iCloud, they were greyed out there too. This is the message I get when I try to open them on my phone (notice the little circle with exclamation point in the bottom right corner):

I also get this in iCloud:

Finally, I have heard I can go into Settings>iCloud>Photos and turn 'iCloud Photos' off and then on again, but when I do that, I get a message that says "This iPhone was not able to download 3,504 full-resolution photos and videos. If you continue, the low-resolution version of these photos and videos will be removed. Options include: Remove from iPhone, or Cancel". I'm concerned I will lose the photos forever by continuing with the action.

Is there any hope? Can I get those missing photos back? There's at least one section of photos that spans almost a year, with photos of my kids, that'll be missing if I can't get them back. :(

TIA for your help!