To try to bridge the gap between OP and some of these replies, perhaps there is a middle ground.
Replies here saying that the timer is already optional are correct; you still get rewards right now if you ignore the timer. However, the presence of the timer (especially in PUGs) still adds some pressure/weight as people still feel that urge to race to the end if they’re close (which might be fine).
Perhaps a middle ground here would simply be that the key owner, upon placing the key in the Mythic slot, could check a box that says “No Timer” and it simply would not show it to any members in the group, and behave as if the timer “expired” at the end even if they beat it in time?
It’d be a minor (but built-in) UI tweak that’d give people like OP who struggle with the existence of the timer a bit of sanity back without impacting any other pre-existing M+ mechanics or rewards. It’d be an affirmation of the key owner “we agree that the timer doesn’t count here.”