I'm sorry if you feel misled, or if you feel like people are being negative (I may have been a bit blunt maybe even a bit defensive).
Its worth noting that the name of this forum is DIY (Do it yourself) Solar. And I truly don't mean to be rude, but the most basic minimal amount of research would've turned up scores of prebuilt LFP batteries, and a bit more research might've turned up a good deal on a prebuilt pack. They have been discussed many times on this forum. And there are a ton for sale on Aliexpress, as well as places like techdirect and probably ebay.
In either case, this is a DIY forum, members here are not customer service reps or sales reps, we are just people, and most of us like the DIY aspect so that tends to be the focus. And from my recollection Will generally mentions in his videos on DIY LFP packs that building your own pack is not necessarily for beginners. Maybe I'm taking your comment the wrong way, but it feels like you came here blaming Will/us for a purchase that you now regret due to lack of research, if this was not your intent my apologies, its hard to gauge tone/meaning on the internet.
Perhaps you should start a thread on the prebuilt battery pack you found so others that might be in a similar situation to you will know about this option. It may be a valuable contribution to the forum and our shared knowledge.
On another note, what makes you regret your decision? You paid $200 less than the prebuilt pack, that's a fair price, the $200 extra for prebuilt is fair also, but you have no idea the quality of components they use in it. If I were in your shoes, I don't think I'd be dissapointed in what I purchased.