I don't want to offend anybody but I feel like it had to be said that one who wants to or already does an extreme sport, because basically/factually that's what diving is, and especially someone who wants to participate in tech diving, it would be wise and best for yourselfs to get in somewhat a decent shape.
But that's not the most important factor, I think it would only be fair towards other people/your buddy to be in an acceptable shape since your fitness could actually very easily affect also other people very quick very bad in a emergency situation, again especially in tech diving.
I'm not judging anyone in particular but when one's even struggling to find a wetsuit that fits because of being overweight (especially since there're suits available in sizes where 2 or 3 small people would be able to practically dive in) I think one should reconsider to participate in an extreme sport)..
Just one scenario: you're at 18m and your buddy gets unconscious. You're like 100m from the shore and you need to get your buddy there ASAP.
It might be after 50m of pushing your buddy you're already in trouble yourself.
I wouldn't dive with a person so heavily out of shape where one could question they are able to save themselves let alone you...