Parmandur
Book-Friend
"We have a lot of strict rules, except where we don't," is 1)not really all that free-form, and 2) not really a selling point.
If "freeform" is desirable, and it is play without rules... why are folks who want it playing with a ruleset at all? Why aren't we playing just by "GM says so"? In practice, the GM becomes a set of rules you have to play with - just not rules written down where everyone can know them beforehand.
"Freeform" is a wonderfully vague term, and can mean almost anything. It does not need to mean "play without rules". It can mean "play without much restriction" - which, interestingly, can be supported with a ruleset - if the ruleset allows what you want, without having to make up a new rule or ruling each time you do something novel, then the result is freeform.
I submit that freeform gaming is better supported by a flexible ruleset than it is with a strict ruleset with many gaps you have to fill on the fly.
Yes? I agree?
The 5E action resolution is a flexible system for judging an action declaration, and either letting it succeed or choosing one of a handful of numbers, and resolving that immediately. This is what I would define as a system, that is flexible.