PLBs are great for their power, relatively low cost, and reliability.
However, there is a reason everyone I know who is in SAR carries a inReach first and maybe a PLB as a backup. It's because how critical two way communications can be in a rescue scenario. It can help either SAR or Flight for Life get to you faster by giving better location information, help them understand the necessary supplies, give updates of conditions (both of the weather, and the patient), etc etc etc.
It's also really nice when you want to send out a campsite location to a friend who is joining later, need words of encouragement from a loved one. Mine came in really handy recently when I let friends know I was going to be delayed returning from a backpacking trip as I had broken my ankle and was moving slow on the way out.
As for the bivvystick -- I personally don't see relying on a phone as well as a piece of relatively untested technology. Plus their monthly plan (required for connection to the network) is more expensive than the base inReach plan and gives you less features. All for the same price as an inReach mini that can be used as a standalone device.
Bivvy plan: $18/month. 10 credits. A credit is used for each text/weather/location update/path tracking.
inReach safety plan: $12/month (year subscription), $15/month month-to-month. Unlimited preset (up to 3 preset) texts. 10 sms/basic weather updates. Additional text/weather @ 50c, location/tracking at @ 10c.
About 90% of my use of my mini is preset messages so the choice is obvious for me. I send presets every couple hours, esp when I'm out solo that just say "All is well, love you. (Yes to Y/N)" to two contacts. Then "At trailhead/camp for night/car. (No to Y/N)". And one preset I haven't used yet "Hurt not emergency. Please pick me up at location."