You are right, not all features of the Photos Library on a Mac are supported on the mobile devices, particularly smart albums and keywords and titles. Most of the albums on my Mac are smart albums, based on keywords, and I have to double the important smart albums as standard albums to be able to see them on my iPhone and iPad and Apple Watch and Vision Pro. And I have to copy the titles to the captions to see them on my iPhone and iPad. A lot of extra work and a waste odf storage, and a source of inconsistent metadata across my devices.
Which system version is running on your iPhone and iPad?
Until iOS17 and iPadOS 17 the following has been working well for me:
- My System Photos Library on all my Macs is syncing with iCloud Photos to my mobile devices.
- I am adding keywords only on my Macs. The keywords are syncing with iCloud Photos to my mobile devices.
- I cannot see these keywords on the iPhone and iPad, but when I search for them in Photos iOS or Photos iPadOS, the search will find the photos with the keywords and show them as a part of the search result. Until iOS 17 and iPadOS 17 the search result had a separate section "Keyword is ...". This separate section in the search result is now missing in iOS 18, but the keywords are still included in the search result for text.
For example, when I search on my iPhone for photos with my keyword "Lightvessel" on iOS 18, I am getting this search result: 
All photos with the keyword Lightvessel are included in the search result for the Text "Lightvessel". Just the separate result category keyword is no longer shown.
On my iPhone and iPad I am using this workaround to add keywords to my new photos: I add the keyword I want to use at the beginning of the caption, just temporary. When I get back to my Mac I turn the keywords in the captions to real keywords. To avoid inconsistent information I am doing all metadata tagging mainly on my MacBook Air and let the metadata sync to my other devices. On my iPhone I am using the caption to add data right after taking the photo, just to keep notes, why and how I took the photo.
I agree with you, that the missing support for keywords on the mobile devices is very limiting.
You may want to send feedback directly to Apple. Here in the forums you cannot be sure, that the developer team will see your feedback. But they will see the feedback we send using the feedback form Photos for iOS - Feedback - Apple