Ohhhh … I was confused, but now I understand what you are saying.
At first, I thought you were initiating the alarm on a downstairs Keypad and the delay enable you to get upstairs before the alarm armed. But then I realized you were initiating the alarm after you were already upstairs, which really confused me. Why would you need any delay set, if you were not where the downstairs Motion Detectors are?
You meant that enroute to the second floor, you travel past the downstairs Motion Detectors, and they ‘see’ your motion (but don’t trigger the alarm because you haven’t armed yet). You also stated that once a Motion Detector “sees” movement, it needs 180-seconds (3 minutes) “dwell time” of no motion, before it reset back to “looking.” That is what you meant by “sensors to clear.” I agree with you on this fact. I found a ‘Ring Support Center’ document (for Generation 1 detectors) that confirms, that, “. . . after a Motion Detector is tripped it will need a three-minute period in which it detects no motion in order to “reset” back to its default detection mode.”
You also said, “However, unless you wait 180s for all sensors to clear they will not arm.” I think you are saying, that if a Motion Detector is still in the “Dwell/Reset” period as the system arms, then now any subsequent motion will not trigger the alarm. I think that is incorrect.
These Motion Detectors are sensors which transmit a Z-wave signal every time motion is detected, then pause for a 3-minute reset-dwell time. So, they detect motion, send out a signal, wait 3 minutes, and then send out a signal (if motion is still there), and then reset for 3 minutes, send a signal . . . and so on, and so on. This link is an example:
In this case, the Motion Detector kept "going off’ tripping every 3 minutes. That implies it was sending a signal every time the reset-dwell time-cycle completed.
Also, another person asked Ring this very question, “Do you really need to wait 3 minutes before being able to arm the system . . . after seeing motion?”
The answer from Ring was, "If you arm your Ring Alarm before the Motion Detector has cleared _ . . . the Motion Detector will be armed and_ will not be bypassed . Ring Alarm is smart enough to know that as you exit your home you may trigger the Motion Detector . . . After you leave your home and movement stops, the Motion Detector will clear as expected and be ready to detect movement. Any motion that is detected at that point will trigger the Alarm ."
This implies the Base Station is waiting for a signal, from any non-bypassed sensor (Motion, Window/Door Contact, etc.), and will alarm once it receives a signal.
I also would find it annoying to listen to my Ring do a 180 second “countdown.” I’d rather have a very short (or zero) time delay set, and just wait more than 3 minutes upstairs before arming the system. But the bottom-line is, you can arm your system from upstairs, without a need for any delay setting.
I hope this clarifies this topic and you find it useful.