Always amazed this pairing still has such a devoted following, especially for action-heavy stuff. My reading habits have drifted over the years, but whenever I check back, I find the richest stories for Steve/Natasha are on Archive of Our Own. The tagging system is your best friend there; you can filter for their relationship tag and then add 'Action/Adventure' as an additional tag.
Honestly, you'll need to sift a bit, because a lot of fics tagged with 'action' might be more mission-based banter than hardcore tactical sequences. I've found the most satisfying ones often have 'Hydra' or 'SHIELD' in the summary. There's an old series by a writer named 'proantagonist' that I think about sometimes—real gritty, cold-war era Steve and Natasha, with fantastic chase scenes. The platform's bookmark collections can also point you to hidden gems.
Don't completely dismiss FanFiction.net though. It's clunkier, but if you search for 'Captain America' and 'Black Widow' and sort by favorites, some of the classics from the 2010s are still up. The prose can be a bit rougher by today's standards, but the action set pieces were often written with a real cinematic flair that newer, more introspective fics sometimes lack.