Oh, your question just solved my idea, I noew re-structure for my blog, move niche's posts to niche's subdomain (eg: posts about Woodworking will move to https://moc.chebinhdan.com). Now its very clear for me and you:
1. Create a new blogspot (https://mocbinhdan.blogspot.com)
2. Add 3rd party subdomain to it: https://moc.chebinhdan.com (updating)
3. Clone the template from main blog to sub-blog. Fix the menus items
4. Create a new page on new blog: https://moc.chebinhdan.com/p/moc-binh-dan.html (landing page)
5. Remove sidebar and heading of the landing page if has them
6. In new blog: Setting > Search preferences > Custom ReDirect > set redirect from subdomain (moc.chebinhdan.com) to landing page
7. In the old blog (https://www.chebinhdan.com) add the new Menu title > point to landing page URL (or subdomain, now they are the same).
8. Lastly: Create a label named "blog". All the posts will has this label. Edit the Menu and add "Blog" on.
Thats it. You could do the same with contact page.
You could check my blog, I've just test with it.
For sample: Mộc Bình Dân => redirect to Tự làm mộc bình dân (landing page)