Would her entering brood or if male senescence explain why shes been so to herself in the cave never showing me but a tentacle at feeding time?
It is too early to tell. Since there is still an interest in food, it may be difficulty adjusting to the aquarium. If an octopus has been with a keeper for several months and the keeper sees a distinct change in behavior, it is usually a sign of reaching what I would call a growth step. My own observations, strictly as a hobbyist, suggest that from hatchling until about 5 months they are very reclusive but will learn to come to eat, from 5 to 10 months is their most social time and around 10 months they start to be less social, eat less and begin senescence. However, the first 2 week to one month in an aquarium usually shows behavior that will be different (sometimes more interactive than afterwards, sometimes eating everything in the tank, sometimes not eating what is offered but eating small things in the tank) than what you will see after it has fully accepted its new home. None do well if they cannot hide and choose when they want to be seen. Sometimes you will not see them sitting right in front of you in the tank (less so with briareus, frequently with hummelincki). Animals that are well acclimated will actually change color, almost as it to SHOW you where they are when they see you looking in the tank.
If you would try to compare them to a domestic pet, the best personality example I would suggest would be a cat. They will do things their own way and condescend to give you some of their time. One ceph researcher,
Joan Boal has made comments about how much easier it is to try to work with cuttles after working with octopuses for many years because they were less independent and more consistent. IME, most octos that survive their first few months in an aquarium will choose to interact in some way. The ones that interact minimally, will only do so at feeding time. Yeti (overly small briareus) is like this but she insists on playing aggressively with someone's hand for 5 minutes or so before deciding to eat. She takes the food immediately but then insists the feeder has to play tug-of-war with his/her hand. She never drops her food during this time but makes it quite clear she expects the attention. When she has had all the attention she wants she just lets go and dines. Octavia (larger hummelincki), after 5 months has always been a sweetheart and very gentle. She seems to like to be petted (a gentle finger tip rub between the eyes, arms, suckers or mantle - as she ages, she seems to only want her suckers rubbed, possibly to help shed the linings) and will come to the glass at other than feeding time and accept/encourage contact for a few minutes at a time. Both my Macropuses (one male, one female) would
not play at feeding time but would come to the glass and squeeze through my fingers (I think my hand may have served as a "scratching post") most every night (at about 3:00 AM). LittleBit (vulgaris) played very aggressively and would come out almost anytime we were in the room. We never quite trusted her not to bite during play (but I could clean her tank without concern) as she was very strong and never learned to be gentle. Others, never interacted much at all but would come out and watch us eat (the octos are in our eating area, something I suggest because you are there on a more or less regular schedule and sit unthreateningly where they can observe you). Thinking back on it, I believe all the briareus we have kept would come out to watch us during supper, even if they would retreat to their dens if we approached the tank.
So, I can only try to give suggestions on helping to encourage a reluctant eater to start eating with some of the tricks we have learned along the way. If this one is male, it should start coming out of its den, initially very early in the AM (just before sun up) or about an hour after the room is dark. If male and entering senescence you should see it pacing somewhat aimlessly at various times. If it is female and not about to lay eggs, anything goes. If she is about to brood she will rarely leave the den but should be showing an appetite until she lays eggs. Right now, excellent water quality, patience and consistency of food and timing on your part is likely to produce the best result whatever that behavior may be.