@junebugalice this is not you. You are in no way at all cruel or heartless or any negative thoughts you have. Your parents are probably worse than you actually realise. Your mum at least sounds devoid of any empathy and is operating almost at sub human level.
That was not about love for their grandson. That was a few things, firstly, it was a tantrum because they want their toy/object/possession fully accessible to them. That object is your son. The card is them saying, you won't stop me trying to get my possession back. I still have control. And that control is re enforced by any string emotional reactions from you, the victim.
The second part is all about appearances...'don't we look like such caring loving grandparents trying so very hard to see our lovely grandson and we just won't stop trying everything.' That's part of the overt performance.
I get the impression your mum is the head of the system and your dad goes along with it. Equally pathetic and dangerous as a flying monkey to mother.
I am not sure of the law regarding harassment. Whether these two could be treated as perpetrators of harassment. It could be difficult because these arseholes will manipulate everyone who will believe them to be the victims. People don't ever want to believe us as the victims, so the advice is often to sit back and let them reveal themselves. Eventually they always do.
You might have said that you watch Dr Ramani. A few new videos popped up today and her advice is always fantastic.
The entire control they hold is knowing they bother and upset you. When you can get to a point of indifference, you're almost there. They will give up if you can consistently treat them robotically. It's difficult this situation as your mum is bloody determined. She still sees or feels your fear and emotions and that is feeding her. Trying to stop those reactions is hard because she is really trampling into your personal space with this card stuff.
Never doubt why you put them aside. Your doubt also feeds them. Your mum knows you so well, she wants to tap into the guilt ridden part of you willing to take responsibility for everyone like you probably had to when growing up.
People who embark on low to no contact have usually always suffered immeasurably. More than they can cope with fully accepting at first.