You want to pool 2 drives managed by software RAID with 4 other drives in an external enclosure managed by hardware RAID, connected via USB?
Yes, this is a terrible idea, as you already suspect. If you even get it to work, it will be unstable and the USB bus will be a nasty bottleneck for data transfer.
This would be a significantly more stable and reliable setup, and offers room for future expansion. In addition, if data hoarding is the goal, you would be able to use an OS like TrueNAS which would give you better control over your drive pool(s).