I like blazers too. Your ideas are good - the thin hoodie underneath, adding casual shoes.
Regarding jeans - if you normally don't wear them, don't stress over it! I've moved away from jeans (meaning blue denim) and am trying cotton pants like these from Jil Sander.
https://www.net-a-porter.com/e.....-cargo-pan
Or look at linen or jersey trousers. Poetry carries jean-style pants in linen (and other pants too - check them out!):
https://www.poetryfashion.com/.....en-cotton-
Even joggers or sweats or gear-type bottoms can make an interesting "high-low" match with blazers, but that's really a matter of personal preference.
I think it might be a matter of some time invested in observation of street style in your area, and online too (Vogue's website for example) for ideas and inspiration.
I like the shoes you show, but I do feel that lighter colors seem fresher style-wise. For some reason, I connect black sneakers as corrective or required for a foot issue. That didn't come out right, I know - please forgive my clumsy wording.
Angie always features such interesting casual and modern footwear - I'll leave it to her for suggestions!