Quick action > mark as read
Add a new 'quick action' [1] to 'mark as read'.
This would optimize the selection using the list and the (existing) option 'download unread', by acting in 2 steps:
1) review list (scroll) and mark unwanted podcasts as 'read' using the (new) option of the 'quick action' button
2) use option 'download unread' to catch all remaining podcasts in one click.
Moreover, as the download uses (quite a lot of) CPU (i.e. slowing down the smartphone for me), it would postpone it to the end of the review (i.e. less interface impact for a smooth selection / scrolling)
[1] Preferences > Display > Episodes > Quick Actions