I have a 105 gen 3 stages on my road bike and have no issues with it. From what I hear 4iiii power meters are nearly identical to Stages. Stages does get a bad rap for drop out issues but with the Gen 3 that should be fixed. The only time I have ever experienced any drop outs on my Stages is when the battery is running low but that’s not the power meter’s fault. If all things were equal I think I would get a 4iiii because they have the Podiiiium which is rechargeable.
I wouldn’t really put the P2M in the same category as Stages/4iiii. The P2M is spider based and is therefore dual sided. You can read all about the dual vs single sided debate. What it comes down to is single sided just doubles your left leg so if there is an imbalance it will read artificially high or low. It could be very accurate or it could be far off depending on your left right balance. With dual sided you don’t have these issues. Because you talk about GRX I just want to make sure you know P2M isn’t compatible with a GRX crankset. You need a crank with a removable spider.
With all that said these are my thoughts. I would go with the Stages if money is the biggest concern. It does have it’s limitations but plenty of people are happy with the product including myself. I would go with the 4iiii if you want the rechargable battery and you think the extra $60 is justified. Outside of that there is not much different between the two. I would go with the P2M if you don’t mind spending the extra money for a nice piece of equipment. For me if this were going on my main bike I would probably spend the money and get the P2M but if it were going on a bike I use for the occassional gravel ride or cyclocross race I would go Stages/4iiii.