Kid Charlemagne
I am the Very Model of a Modern Moderator
It might be easier to just write up some maneuvers and give them a DC and base stat, with some DEX and some STR.
That pretty much my plan.
Edit: mostly I try to gate as few actions behind die rolls as I can.
I also agree with this - it solved a lot of my players problems when I realized climbing mostly didn't require rolls unless its particularly dangerous.
Not that it necessarily matters in a 5E skills context, but weak people can't do parkour, just like clumsy people can't.
This is true, but thieves need to be able to climb walls!
"Parkour" could be imagined as an overarching challenge that is divided into specific obstacles, the declared tasks for which may or may not call for ability checks as per the normal rules for adjudicating actions. Strength (Athletics) checks covers "difficult situations you encounter while climbing, jumping..." (Basic Rules, p. 62). Dexterity (Acrobatics) covers attempts to "stay on your feet in a tricky situation..." (Basic Rules, p. 63). As always, only the DM calls for checks after the player declares an action that has an uncertain outcome and a meaningful consequence for failure.
So, in a rooftop chase or tricky exploration of a section of a dungeon (for example), you present a "parkour exploration challenge." That challenge has a set number of obstacles (say, 3 to 5) that are individually presented to the players via the DM describing the environment and asking "What do you do?" Examples might include a sheer wall with few handholds, a narrow beam connecting two rooftops, a low fence or barrier with sharp protrusions, etc. The players will describe how they try to get past it at which point you can decide if an ability check is required according to the rules for adjudicating actions. When the "parkour exploration challenge" is complete, you can assess the overall success or failure of the PCs based on how they managed the individual obstacles and narrate accordingly.
This sits within the existing rules set and does not require any house rules or the like.
This is a really good way to look at it - I am definitely wanting to keep things within the existing rule set and not get too outlandish. I have a challenge of this sort coming up in the next couple of sessions that has kind of crystallized my need to create some more concrete guidelines.