So I've read most but not all of the thread and I can see both sides, though I think some people could have been gentler with how they phrased their opinions...
Basically what we're talking about here is boundaries.
And first off I'm going to point you in the direction of a brilliant book (series for every scenario), called Boundaries by Dr Henry Cloud and Dr John Thompson. I have no vested interest but I'm a coach and I have used their books as tools in my work and in my home life A LOT!
One analogy they use is that boundaries are like a fence around your property with a gate. You get to decide who comes in and out.
For some people they don't trust the fence and gate, so they build castle walls, complete with drawbridge, moat and possibly a fiery dragon to keep people out!
However, you can make something so secure and well protected that no one can get in and that can have negative consequences. I think this is what you're worried about with regard to your DS.
A couple of hypothetical scenarios for you...
A) You continue as you are, GPs randomly call up on the iPad without any agreement or concern for timings etc. Let's say this carries on going til DS is 13,14 yrs old. DS now has the idea that when GM calls he has to answer no matter how inconvenient it is, as he has never learned a clear boundary that phone calls are for the receiver's benefit and can be at their convenience. Now DS wants to share about their new love interest (assuming they still have a good rel with GM)... but this person is someone GPs don't approve of (fill in the blank...different race, background, different gender to expectation)...GM expresses shock/horror etc and says no this won't do at all. DS talks back. GM cuts him off as he isn't doing what they approve of (I think this is kind of what you alluded to that there is no room for alternative views or choices?). Regardless DS is now hurt and upset by seemingly loving GPs reacting negatively to him having an opinion and a boundary about what is okay in his life. Now...unfortunately you have to step in to protect your child, and probably have the same arguments with the GPs as you had before you went NC.
B) You take this opportunity to sit your DS down and explain that phone/video calls have to convenient for the family as a whole, so now you're going to ask GPs to schedule calls. You also let GPs know you will be present, if out of sight.
Here you start, very simply and gently to set up the idea of healthy boundaries. As DS gets older you can start to talk about what they need to do if someone says something that makes them feel ick or sad inside...come talk to mum and dad, and let's work out what your needs are here, let's work out what kind of a boundary is going to help you feel better, safer, more confident in future, and let's find a way to do this in a respectfully assertive way.
C) You choose to do what some PPs have suggested and go fully NC.
Initially DS may be confused, wonder why GM doesn't care any more, but not enough to ask, so it leaves a slight nagging doubt that maybe they did something wrong. Maybe you explain that the GPs upset you and you have chosen this route. Just check in with yourself, what message does that send? If you upset your child later, is that their first reaction, to go NC because that is what was modelled and they don't know any other way of setting a healthy boundary? I think you are also concerned that down the line they may be upset with you for not giving them the chance to have that relationship with them.
I am NC with my brother for about 5 yrs now, so don't think for a moment that I am questioning that aspect. I just think this could be a good ongoing opportunity to teach your DS and eventually both your DCs about setting and enforcing healthy boundaries little and often. I understand what people have said about them being toxic, but your LO is too young to understand that, and at the moment their ability to introduce that toxicity into DS's life is very limited indeed so I don't think you are doing anything wrong by allowing it. It is your choice ultimately.
You are also entitled to change your mind one way or another down the line, in order to work out what is healthy, what you are willing to allow, and what is safe.
I have had to work extremely hard with my parents, especially my mother, on what are acceptable boundaries, both with me and my son (14 yrs). Unfortunately for her now, her constant overstepping, not listening, subtle guilting, shaming etc means that my son is unwilling to spend more than the briefest time with her. We go to lunch with them, have a pleasant, fairly superficial, but safe time, and then we leave when my son has had enough and wants to go. I still gate-keep a fair amount as my son has some SEN, predominantly anxiety.
Someone did suggest I went NC with both my parents, to see if that helped my son. I considered it, but my dad is 76, and for me I couldn't live with cutting him off in order to distance my mum. I did set stricter boundaries with her, I do sometimes threaten to hang up on her when she won't let something go (but I do know it is only because she cares and is desperate to help...but her controlling nature makes it hard and A LOT!), and we have upped and left when she wouldn't calm down or let something go (dog with bone), but I try to do it calmly and firmly, and then explain to my son why we needed to do that. My hope is that my son will have a much much better understanding of how to keep himself safe as he grows up than I did, and also know that I am there for him and will protect him, even from my own parents if necessary. It's harder, but I think it is worthwhile. Over time she has got better at self-policing, and she knows much more clearly what I will and won't tolerate, and the same with my son. I don't think she will change her underlying beliefs that drive some of the unhealthy behaviours, but that's not surprising as she is 70, and has no interest in therapy or improving her emotional intelligence.
Sorry, that's long and I didn't even say if you are BU or not! I think it's more complicated and nuanced than that, and you and your son deserve better than a simple yes or no.
As for sending the text, you could simply write, you are busy with activities and holiday plans and whilst you accept the GPs calling, you would like them to schedule a regular time so you can build it in at a convenient time. You would suggest xy day and time. If this is not acceptable then you will be removing the GM's email from those which will be accepted by the tablet for incoming calls.
HTH x