The SSD Bracket gives you the ability to mount the SSD’s but you still need something else for data. This will probably be different based on the version of the HL15 Beast you pick - Asrock vs Asus ProArt.
With either build, you could buy a second LSI HBA card. 45HomeLab recently started selling the LSI 9400-16i on their website. In my opinion, it’s a good price for the HBA - especially knowing it’s authentic. Unfortunately, eBay is full of cheap knock offs.
With the Asrock build, you might be able to use the OCuLink ports for the additional SSD’s. I was able to do this with the ASRock Rack X570D4I-2T motherboard. The BIOS had a built in option for SATA over OCuLink so I only had to buy a break out cable. I don’t know for sure this feature is included with the ROMED8-2T motherboard. Hopefully, I will soon as I’m in the process of giving my HL15 v1.0 the 2.0 treatment.
You also mentioned NVME drives. OCuLink is again an option certain nvme drives like U.2 or you can go with the M.2 carrier card in a PCIe slot. 45HomeLab recently started offering the Startech DUAL-M2-PCIE-CARD-B option on their website. There’s a quad version of this card too you can get on Amazon or Ebay if you look for QUAD-M2-PCIE-CARD-B.