Hi there. Just for you, I have tested bookmarks and they do work, so that is a partial solution. However, I do find the look of Mumsnet New a bit irritating. I will probably use it for the time being until Tech can fix the other issue.
What Sootikin said ("on my Threads I'm On page I can click on the name of the most recent poster, and instead of being taken to that post I'm taken to the start of page 1 of the thread") is exactly the issue I have too. If you could fix that, I could live with the pages.
Having said that, I still prefer everything on one page - on a long thread, it means you can scroll up and down more easily and it's nice to be able to do a Ctrl F for the OP's name and skip to their posts if it's something where you want to know what actually happened. 1000 posts really isn't that much, you know. Another site I use allows people to see 1000 posts at a time but some of the actual threads are 20,000 posts long! It doesn't seem to impede functionality at all (obviously the poster numbers are smaller than here, but then the infrastructure is also less advanced, at least I assume so).