How do I resolve iCloud sync issues for Safari bookmarks between iPhone and Mac?

Hi. I'm totally tearing my hair out over what should be a simple iCloud sync. I have an iPhone 11 running iOS 26.1 (fresh install as iOS 16 was also causing a problem). I'm on a 2018 macbookpro running OS 15.4.1. I have the latest Safari on both. I run 1 iCloud account and am signed in on both (and also online for the sakes of 'seeing' the sync - I would normally not have this online access selected). Ok, tech specs over. I can't sync bookmarks. Tabs sync (slowly) but bookmarks absolutely refuse to sync. I've deleted iCloud on my iPhone and re-signing in, and I've purged the cookies etc on that device. I've also stripped out every bookmark and tab and now just have a couple of test folders and tabs. On my mac I've saved a copy of all my bookmarks and stripped them all off Safari. And now add, as per the iPhone, a couple of test folders and tabs.

When I add a folder to my iPhone it runs a bit glitchy, sometimes I need to repeat the process. Then when I add a website as a bookmark it appears as a grey box with no description and no link - just a a dead grey box. I have toggled iCloud on and off, turned the phone/mac on and off, shut don Safari a whole bunch of times. (Yes I have good wifi).

My aim is to reload my freshly organised folder nested structure bookmarks back onto my mac and have that replicated and sync'd on the iPhone.

A ? is via the online iCloud; accessing a quick peek at bookmarks via the Data Recovery option I can see the beginning of what is estimated to be 14,000 bookmarks. Is this the blockage? How do I delete these if so?

Can anyone help? Many thanks. I'm literally on my knees with this. I'm even contemplating Chrome.

[Re-Titled by Moderator]

Original Title: iCloud Sync Problems: Safari Bookmarks over iOS 26.1 and OS 15.4.1 on clean install / clean Safari. Tags sync but not bookmarks. Bookmarks are greyed out on iPhone. I can't even add new. Help please!