Your iPhone has a sort of visual programming language built in, called Shortcuts, that can easily batch resize photos (amongst so many other things)
There is a really active Reddit community that discuss them at length and you'll probably find a boatload of suitable scripts already done and quickly installable for you. I'm using one I found and modified but others such as this one look good:
It's a small script file that hooks into the photos app such that you can eg select multiple images from your reel, hit "Share", choose the shortcut and answer its questions, and have it batch resize all of them to eg X pickles wide by some automatic proportional width, and save them for upload. No apps, no ads, etc just built in iPhone functionality
Shortcuts are amazingly useful if you want to nerd out programming your own, but also you're nearly guaranteed to find one that someone else has done
Appreciate the argument about "DIYnot should just allow larger sizes" but one has to stop somewhere - server memory (it's not about disk space, which is long term memory, it's about RAM - short term memory), is a shared, finite resource and the forum software imposes limits to keep things reliable for all.
An iPhone 16's 48 megapixel 32 bit colour image will chew up between 200 to 400 megabytes of it depending on the strategy for processing it (that's the uncompressed size), and it isn't really reasonable for a popular forum to allow a single member to consume so much resource, however temporary. If you wanted to foot the bill for a dedicated server to resize images, perhaps they would reconsider though!
