You ask a good question and I am interested to see how it works out for you. To fully understand where you were going I ran your post through AI for clarity. When expressing power in terms of watts, it seems easier to compare different charging solutions. The AI points out that it comes down to the capabilities of the Mutiplus inverter.
Keep in mind, you also have 600 watts of solar and nominally you will get 350 to 450 watts from it. For the math, your battery is 300a x 12 vdc is 3,600 watt hours. Going from a 20% to 100% level charge requires 2,880 watts. If your solar generated 400 watts, you can recharge to 100% in 7.2 hours. Your 50a B2B at 12 vdc is 600 watts, combined with 400 from your solar is 1000 watts and your 20% to 100% charge is complete in 3 hours, not too bad. These are smart chargers, the last few percent will take longer.
I’m specifically interested in seeing how learning how the 50a Orion DC-DC charger works out. The first Renogy 50a B2B charger I installed overheated and had to be replaced. For the replacement unit I jumpered it to only run at 25a.