BD has a few toys that are pretty much trainer; Winston's Tail, Ika and the new unicorn horn Mystic. all of them are long, tapered and reasonably girthy. Any of them should work well for you.
Non BD there is SquarePeg
https://www.squarepegtoys.com/ them have a lot of very nice toys for training. I have a Slink and i love it.
there is also
https://www.siliconenozzles.com/main.sc they make various different clean out nozzles in all range of sizes and lengths that can be ordered with or without a bore. I have a few nozzles from them and have been very happy with them, but I dont have any really long nozzles. I have been eyeing a few, but have yet to order one of them.
As for general advice about depth play, practice, lots and lots of practice. If you have smaller long toys i would recommend practicing with them until you are very comfortable taking toys deeply. then get thicker and thicker toys until you are comfortable with each size.
Rex is a bit of a difficult toy to knot. The toy is long enough that if you do manage to take its knot, if the tip isn't lined up with where it needs to go. when you do knot it, it will jab you very painfully. i can tell you this from first hand experience. Always warm up before attempting to knot Rex.
Rex is a pretty fun toy once you get to used to it. Just takes a bit of practice.
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