As a quick-and-dirty estimate just assume that an injection interval of one half-life means the peak serum level is double the trough. Assuming normal SHBG — so that free testosterone is also reasonable — a decent monthly-injection protocol with testosterone undecanoate might have a peak serum T level of 1,000 ng/dL and a trough of 500 ng/dL. So no, you should not experience a crash if you obtain these results. The caveats: the half-life of T undecanoate varies depending on the carrier oil and individual; the long half life means that stabilization can take months; lowering the dose can lead to side effects that take time to resolve; test SHBG so that you can track free testosterone, which is more important than total.
My guess is that in switching to testosterone undecanoate you could lower your dose by at least 5 mg T per day. Rounding off a bit, this would say that you could inject 200 mg TU every two weeks or 400 mg per month.
Here are some positive anecdotes about lowering the dose.