There's a lot of info on not adding batteries later, but once a battery is used, the battery is aged. Kind of like a pro-tennis doubles star coming back decades after retiring. The partner would end up carrying the load.
This link explains why batteries can't be wired any which way in parallel
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank. Adding batteries later is kind of like wiring in method one. The batteries with the least resistance, the newer, carries the load, and kills it.
I really replied because I started off with eco-worthy. I got the bait and switch a couple of times. I ordered directly from the website for a pre-built combiner, and the order went through in stock, but I got an e-mail a couple of days later saying they would ship a smaller combiner right away or for a mere $50 extra, they would ship directly from China. I cancelled my order. I tried again with six of the 120 watt panels, and again the order went through in stock, but again I got an E-Mail saying that the panels were backordered. Again I canceled the order. Perhaps in Amazon, you won't have those problems.
The Eco-Worthy kits do not come with everything you need for a solar build. There's nothing wrong with that, just realize you will need to buy things to properly install it. The kits often come with not enough batteries, no wiring, no circuit breakers, no brackets, no mounting board, etc.. all of which add up. These solar kits that are marketed are almost all like this. Gives someone the impression, buy this kit and you'll have everything you need, but the stuff I mentioned as missing costs at least as much as the kit itself. Your kit you linked comes with a lot of stuff, but proper inverter wiring is missing, also how to mount the inverter including a power board, and also depending on the install, because the cables are prebuilt, you need to drill the wholes to the roof bigger than just sticking wires through and making the cables, which making cables is easy.
None of what I mentioned is insurmountable, but you'll need to have some money set aside for that. There's so much more that goes into this. I found I needed about a tube of Dicor Self Leveling lap sealant at $10 a tube for each panel I installed. If not getting more batteries now is a problem, then plan well.
As far as batteries, you need to measure your power usage. IMO if you use very little power, you can get buy with two batteries. With two of the batteries you mentioned, I could usually run my propane heater all night, the fan draws a lot of energy, in my 35' RV, but if the temps dropped below the 40s, the fan comes on so much, it'd drain the batteries to less than 50% which gives them many less cycles.