You will overcome this plateau! It takes time and variety of tonguing work.
My single tonguing speed is about 120 bpm/sixteenths for long passages, and my max is maybe 132; my present goal is 144, so I understand.
I've found what works best for me is playing fast articulated sonatas. I have a book of "12 Sonatas" by Johann Mattheson that I like to play, and they range in speed from very slow to very fast.
Rather than learning one entire sonata, I can jump around from movement to movement, progressively increasing speed. Several movements are the same speeds, and so in the time it takes me to learn two movements of two sonatas at, say 112, my tongue speed will improve immensely and by the time I've mastered them, I'm ready to move on to 116. Playing sonata's like this forces me to practice rapidly tonguing intervals, too, instead of just one note (equally good for the embouchure if you focus on clear tone on each note).
Sonata's are also effective because even if a movement is straight 16th's start to end, there is usually variety of articulation (tongue 12 notes, slur 4; tongue 4, slur 2; etc), and breaking it up like that gives the tongue a chance to recoup so it's easier to make it through an entire movement without slowing down.
I'd suggest even starting with pieces that are slower than your present cap. Learning to play an entire sonata movement tonguing 16's at 96 bpm will increase your max tongue speed too.
3 weeks isn't really a very long time to develop a muscle like this. It's taken me 11 months to increase my consistent speed from 16ths at 100 bpm to 120 bpm. But I also find that after I learn a piece, if I take a month away from focusing on my tongue and work on something else, when I come back to my tongue the muscle has had time to rebuild itself stronger and I can tongue faster even if I haven't practiced tonguing for a month. Keep in mind that the tongue is a muscle and needs time to rest and rebuild.
JS Bach, CPE Bach, Mattheson, Handel, Telemann, Quantz, Vivaldi all have good sonatas and concertos and fantaisies and suites to practice this way. You can probably find a bunch of them on imslp for free.
Hope this helps!