I haven't caught up on 15 pages so this may be redundant, but...
I have come to the conclusion that the way to make "player actions matter" is to have fewer rules. I know the 5e motto is, supposedly, "Rulings not rules" but 5e still has a lot of rules. And while that applies to some areas (spellcasting, for example) more than others (social interactions with NPCs, for example) the result seems to be that DMs/players think there are supposed to be rules...because, Look!, there are all these detailed rules for this other thing...so when the rules for one thing are intentionally vague people still try to figure out and apply RAW.
And I'm a case study of this phenomenon. I'm finding that when I GM Shadowdark, my current favorite RPG, I am always looking for ways to say, "Yes, and..." to crazy player actions. I think it's partly the result of intentionally sparse rules, but also partly the result of the intentionally sparse area descriptions. Whereas with 5e I was always double checking if there is a rule, and re-reading the area description to see if that case is covered, and trying to do the "correct" thing. Something about Shadowdark freed me from that straitjacket.