When combined with nontrivial amounts of solar, DC-DC charging is a decreasing return expense. Normal isolator charging would do the Bulk-style heavy lifting adequately and much more cheaply.
Reasoning: normal isolators raise the bank to alternator voltage (Valt, something like 13.8v, maybe higher). The primary value of DC-DC charging is that it DC-DC upconverts voltage to reach Vabs (Pb) or to induce top-balancing via overcharge (Li). Let’s look at the cost/benefit, assuming the 300Ah lead bank AGM absorps at 14.4v.
a $25 constant duty solenoid would get the bank up to Valt, at which point 70% of the current needed to charge Pb and 100% of the current needed to fully charge Li has already been provided. Solar would have to lug voltage up 13.8v → 14.4v for Absorption/balancing.
a $100 voltage sensing relay would do the same. Cost more for the autoswitching magic and possible crude overcurrent protection via internal autoresetting breaker
a $200-400 DC-DC charger (CTEK, Renogy, Sterling, etc) would bring battery voltage right up to Vabs (14.4v).
Solar would just handle the high voltage, decreasing current Absorption proper.
What we are paying $100-$175 extra for is raising voltage from 13.8v → 14.6v faster than solar would have done it alone.
Exceptions (where solar + DC-DC can be valuable):
underpaneled systems that would struggle to make the 13.8v → 14.6v slog on their own in normal insolation. Going from 800w of panel down to 200w + DC-DC can save hundreds of dollars under optimal use
using a smaller DC-DC charger to limit the amount of power Li or large AGM banks can pull from the alternator
people who drive for hours at night (delivery folks, etc)
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