What waterproofing rating is sufficient for a navigation device for touring? I am exploring the possibility of using my phone as the main navigation device for touring. The phone (Pixel 2) is IP67-rated and I was planning to use it without any additional waterproofing, just attached to a Quad Lock handlebar mount.

I am now wondering whether serious rain is going to kill it or at least render temporarily unusable (touchscreen issues come to mind)?

Since I don't really want to find out the hard way, I thought I'd ask here in case any of you have relevant experience.

For comparison, my Edge 810 is IPX7 and has been fine in all sorts of conditions.

As an alternative to putting my main phone at risk, I am contemplating getting a cheap IP68-rated rugged phone (like BV5800 Pro). Not really sure whether it's likely to fair better (higher ingress protection rating) or worse (presumably cheaper construction).

P. S. Having looked into this further, I came across MIL-STD-810G. There are quite a few devices that advertise this, but I am not sure how much attention to pay to it (given that it's self-certification just like IP ratings are)?

P. P. S. There are phones that are specifically advertised as being operable with wet or gloved fingers (for example CAT S41). Not sure whether this in any way relates to MIL-STD-810G, or how much attention to pay to it. Would love your input.
Easy bit - Touch screens don't usually work when wet, you can easily test it, sprinkle some water on the screen and see how it goes. 

Because phones are high volume consumer goods designed as life style toys, not outdoor equipment, as far as IP ratings, I personally believe the claimed IP ratings are based on marketing need for a bigger number than the other guy, tempered by engineering squealing that IP99 is not a real rating, and Finance insisting at that price point they have a budget for X returns out of Y sales. The exception are devices sold as ruggedised.  Read the warranty fine print - I bet it excludes water damage.  

IP67 is 1 meter for 30 minutes, but the test is done in factory conditions on a new phone, not after they are a few years olds and lost elasticity, and the user has contaminated the seals with dirt and dust. I had an IP67 phone with a single o-ring seal for the battery cover. I was happy to get it wet in the rain, but would not have left it on handle bars for hours in driving heavy rain while riding. Driven rain on a fast down hill could exceed the pressure of 1 meter of water.

I would also be concerned about "solar driven" moisture entering the phone. It gets wet, sun comes out and shines though the water onto the phone. The water close to the phone surface heats and vapor pressure increases, driving water vapour (not water droplets) into the phone. The expanding and contracting of air inside the phone from changing temperatures can also cause a negative pressure that sucks water in. 

Although the risk is small, if you plan on riding with the phone on the bars in all weather, put it in a case or accept you might need to buy a new phone. But in a case, the touch screen may or may not work - you would need to test it.