My experiece with Simandl is that it is great for left hand, but doesn't really tell you how to solve the difficulities of the bowings. At the given excersise, you will benefit from planning your strokes (whole bow, at the frog, at the tip, in the middle). I would play it slower and would focus on playing the quarters with whole bow, eights at frog / at tip - at whatever bow end you find yourself after the long stroke - and half notes with whole bow, but slower moving than on quarters. Generally, you want to find a way that lets your bow move in a constant speed even though the note rythms vary. This is to achieve a consistent dynamics and tonal qualities. When you learn that skill, you need to learn also the opposite - play the same note lenghts in the same dynamics while varying the bow speed - because, as in this quite difficult excersise, it is not always possible to play in constant speed.
Write down a 'bow plan' like this one:
View attachment 7055463
f means frog,
w is whole bow,
t is at the tip. (Do find the right shortcuts - I'm improvising these, having been taught the Czech ones.) The two signs above the stave are commonly understood signs
n = 'start at the frog' and
v = 'start at the tip'.
In bar 3, you need to gradually transfer from the tip to the frog while playing eights, which is an excersise on its own.
Until you can do it, abandon the metronome, follow your own tempo. It might take long time! You will learn it faster when you go slow but don't let any problems unsolved.
If you find the above useful, you would perhaps benefit from a different book. Gajdos/Sevcik comes in mind. And, of course, from a teacher.