A pullbuoy will give you the feeling of where your hips want to be
then you can ditch it and you'll remember the feeling and start to know when your legs are dropping.
Fins imo are no good, it's imprinting kicking to keep the lega afloat which is the opposite of what is wanted for TI
the aim i to use a 2bk for balance and as an aid to rotation and streamline.
Of all the styles TI is the best for learning balance and streamline
the difference can be seen at any pool where the good swimmers are effortlessly streamlined with a minimal 2bk and the mediocre swimmers are kicking like crazy to maintain position
i can spot kick dominant swimmers a mile off now.
(that's not to say kicking is bad and shoulder driven sprinters will be using massive 6bk's etc but the best distance swimmers are not kicking to keep the legs up it's an aid for balance / anchoring & rotation.)