I was in this situation with my DH's parents. It was soooo suffocating and DP (now my DH) was incapable of standing up to them. Like you, I was expected to spend my birthdays with them. DP's sisters would harass him if we hadn't seen his parents recently (no doubt because his parents complained to them).
It got to the point I stopped answering the phone because I couldn't cope with having to come up with on the hoof excuses for the next 'family' get together. 29 years later I still don't answer the phone.
But it came to a head when my parents came to visit me (from another country) and my DF was in very low spirits. DP's parents phoned every single day badgering to meet up with them - getting angry with DP. DP and I were just boyfriend and girlfriend then - and my parents found the whole thing weird and too much. But my whole family tolerated an awkward get together that none of us wanted to be at because DP was so incapable of dealing with it and so obviously miserable.
I absolutely lost it. I realised DP was incapable of being sensible about it so I said I would only l, from then on, see his parents once every two months, and the rest of the visits were his burden alone. Never ever would I spend my birthday with them again and I didn't care what he told them.
When DC came along, we spent their birthdays as our family unit, then had a second more casual get together for the rest of the family. I refused to see them until I was ready to after the DC were born. I told my DH I didn't want to know about their demands, or how they felt about my decisions . It was his problem to deal with and I insisted on being kept oblivious of it all. His family; his issue.
Honestly, if FIL hadn't died, and MIL didn't develop dementia, and the behaviour had continued, I'm not sure my marriage would have survived it. It is soooooo hard.
And, OP, if they had popped into a family holiday abroad, like yours had, I'd have gone crazy.
Get off the WhatsApp chat so you can't be involved in plans at all. That's DH's problem. Tell your DH the limits of the time you will share with them, then he deals with it. Tell him you're not interested in the politics it might generate. Again, his problem. Warn him if he 'blames' you then it will create even bigger issues in the family that will make things worse and which HE will have to fix. By himself. Set your boundaries and stick to them.
You married your DH. You did not marry his parents. He might not mind them around but you do - whether he thinks that's unreasonable or not is completely irrelevant. If he wants to spend all his time with his parents, let him, but make sure he understands that could impact n your marriage. His choice. The alternative is he is asking you to accept something that is making you stressed and miserable - I.e. putting their unreasonable demands ahead of your mental health. That is not OK.
It sounds like this little stalking venture is your watershed moment. Ask your DH to read this thread. It is possible he is clueless about the level of damage being done, by you being denied space to breathe and live life privately and on your own terms.
Good luck. I know how horrendous it is. But you are not a mean person for feeling this way. No matter how nice they are, they cannot relentlessly piggyback on your life without it at some point becoming too much to cope with. Your DH needs to know this.