If you want that Chess Records / Willie Dixon / Big Crawford blues/slap tone you need gut strings. Just my humble opinion.
Plus, guts are
the sound for slap bass, IMO. They have been the slap bass standard choice & sound since slap bass technique was dreamed up.
Some cheaper alternatives that will get you
part of the way there, sort of, are Innovation Super Silvers / Silver Slaps. They have a boomy, plummy pizz tone and slap very nicely - but all nylon-based strings can be a little twangy when slapped. These strings last a very very long time also.
My favorite combo is Velvet Garbo Light E&A / gut D&G - a very versatile pizz/slap setup (not bueno for using one of those "bow" things though

) .
Also - your steel strings shouldn't be breaking on you so easily unless they are cheapo crappo steels or something. You should have your bridge and nut grooves looked at to make sure you don't have any sharp edges snagging the windings. I hate steel strings for slap though, maybe you are just being rough on them.