Multiple archive displays

Hi
I am still learning WordPress and am not a developer so please bear with me ??
I have a menu with links to some of my most popular categories. When a link is clicked, it brings up all posts within that category. They are displayed in an excerpt type manner with my featured image, a text snippet and a ‘read more’ link – all good.
I also want an archive page which displays all my posts, by year and category, as thumbnails using their featured image. I achieved this though only later discovered that it also over-rode my excerpts – not good. So now when a category link in my menu is clicked posts are displayed as thumbs instead of the excerpts I really want.
In short, I want an archive page where all posts are displayed as thumbs AND a menu where category posts are displayed as excerpts.
Does this all make sense?
Any advice greatly appreciated.
Site not live yet so no link sorry.