I would rate Marder have better protection than BMP's mainly because it does not have to swim. Thus it can devote more weight to armor. 46% of the weight of an IFV or APC is typically devoted to armor protection. Marder is roughly 33 metric tonne thus it boasts some 15 tonne of armor protection, it might protect it to most Russian heavy machineguns and sharpnels.
Ukrainians will use it like their BMP's minus direct river crossings. They will likely put Javelin there or their own Stugna ATGM's To "even" the odds when encountering BMP's or BTR equipped with ATGM's.
Ukraine already received Milan ATGMs from France and Italy, they can use those without any modifications.
Is there any evidence that ATGMs fired from tanks and IFVs have seen any use in this conflict by either side? There's been ample opportunity, all BMPs and most T-64/T-80/T-90 and T-72B variants seen in Ukraine should in theory be able to do it.