I have the Kraken x62 from NZXT for the i9-9900K. have three cases of it with E-ATX mainboard each in the open configuration (Laudwerk inserts removed). Cooler built into front, blows air into case. Push works well and has plenty of space for push-pull. Digitec published an article on Push Pull a few days ago.

No problem, 1x 5.25" is available.

6x are included, the others can be bought. Review: https://www.gamersnexus.net/hwreviews/3177-fractal-define-r6-case-review-mid-tower