You said you are (or were) always texting and chatting. Things he tells you about their sex/romantic life:
"I know she's found [his asexuality] hurtful and frustrating and she's tried to change him sexually."
I think perhaps this is fair to say generally to an opposite sex friend, but I hope to god he didn't go into details of her attempted seductions. I would be mortified for you to know that if I were the wife.
"He did tell her from the beginning that sex was off the cards and while he loves her and his body , the physical acts surrounding sex with women repulse him.
It's like he enjoys the visuals but the acts turn his stomach."
Even if I had a straight male bestie I don't think I would want to imagine he and his wife in bed while learning about his specific arousal and thought process to this level.
"Her dealbreaker was always kids. What's saddest there is that she has accepted that she won't be having kids which is just enormous I think."
I would accept and appreciate this pity from someone I had confided in, not someone my husband had confided in on my behalf.
"He absolutely loves her although he recently told me that his wish for this year is to get to know her better despite being together for years (you used a confused emoji here)... as well as some other hopes for the year too."
"She would like marriage but he is very wealthy so I know he is a bit worried about that in that she is not."
It's a tiny thing but I don't know if I would appreciate my husband's female best friend using a slightly judgmental confused emoji about our marriage. From the above 2 quotes (in context with all else he's telling you), there are 3 people in this relationship, not 2.
I'm actually struggling to understand what the issue is here. You said he has respected her wishes to cut contact (especially 1 on 1 with just you 2) down a bit. You asked to loosen contact during your own marriage as well, when he fancied you inappropriately (despite you presumably also being just as frumpy then?). So surely it's OK to do the same for his partner.
On the other hand, if he still wants to meet you 1 on 1 all the time and constantly text you, go ahead but don't be surprised if she dumps him. Respectfully it sounds like that may be exactly what the 2 of you (your bestie and you) want. I don't mean this as an accusation but a neutral observation.