Omg this is so true! I'm an INTJ female and I recently met an ISFJ counterpart! He notices all the little details and makes me feel very safe! I can just walk with him and talk about things going through my head and he always listens while taking care of the SE stuff I don't notice. He's also very intelligent and we share a lot of common views but also different views as well. It's very interesting. I've noticed that he likes to be reassured and that he can be very sensitive at times. I tend to use logic to reassure him but i've seen that this helps him calm down a lot of the times! But I have to be careful not to hurt his feelings. ISFJ's are very pure and warm people even though they don't like talking about feelings as much, which is perfectly fine with me!
They are lovely despite being so boring these ISFJs. Like your sweet grandparents. You can’t discuss many ideas with them or criticize them bluntly and their reasoning doesn’t make a lot of sense. But they care too much for you (and other humans) that you can’t hate them. This is how simple goodness and compassion can prove to tame a cynical INTJ villain... or to be absolutely used and abused if the ISFJ is not careful.
That part where you said not to criticize bluntly is very true and important. As an ISFJ I tend to play it cool while feeling my heart crushed into pieces that one time my professor really talked me down about something.
I don’t think it’s a great combination. I’m an isfj and I find intj to be boring and although they have plenty of substance they can lack depth something’s just missing.