Sensitive areas unreachable by Aneros toys?

... I felt that there is another area right above where the tip of Eupho Trident can reach and it seemed to need more stimulation. ...If I move the Billy during the contractions more pleasure came from the entire contact area, and lots of fluid also came out of the penis. I also felt that I needed something even longer, wider, and more texture than the Billy to get more stimulation.
What is that spot I found?

Most likely the spot(s) you are referring to are the seminal vesicles. These glands produce the majority of the fluid found in semen. They sit above the prostate and curve themselves around the back side of the bladder. (see diagram)
When properly stimulated they too can be milked just like the prostate gland.

Did I just open some secret door and my ass now not only need Aneros toys but also dildo/penis?

I don't think this is any real secret, it is just you are becoming more intimately acquainted with and knowledgeable about your own body from your anal play experiences. Whether you choose to further concentrate focus on this area is totally your choice. As you've already noted these areas are not typically stimulated by the Aneros line of massagers, thus employing other tools may be necessary.

Could someone recommend a dildo that is a (smaller) step up from Lelo Billy?

The best tool I can recommend for stimulating/massaging this area is the Njoy 'Pure Wand', it is long enough to reach up to the top of the vesicles for massage/milking purposes and it also functions beautifully as a prostate milking tool.
Good Vibes to You !

@rumel
Thank you for the information.

After the experience last night, I feel the urge to be stimulated is now coming from a wider area than before, and lots of fluid built up waiting to be milked.
It seems that some more nerves were sensitized similar to the re-wiring process (or maybe the re-wiring process is still ongoing...)

I have found that in recent sessions, oftentimes the areas right around my testicles, to the base of my penis feel 'electrified', like there is pleasure running along a couple of hot wires circling that area. could these be the seminal vesicles that are charged and aroused?

I have found that in recent sessions, oftentimes the areas right around my testicles, to the base of my penis feel 'electrified', like there is pleasure running along a couple of hot wires circling that area. could these be the seminal vesicles that are charged and aroused?

I get the same electrified feeling you described, and I think they're linked to the penis.
With the technique I described in this post:
https://community.aneros.com/community/general-discussion/male-chastity-device-questions?p=148916#post148916
I can intentionally trigger SuperT by simulating near these areas.

@bbs999
oh wow dude thanks. Something new to try in the next session!!