For external, I think a 4 bay HDD enclosure with USB 3.0 connection can do the job very well. For $1k, 4x 6TB Seagate IronWolf should cost less than $750. A USB 3.0 card cost about $30 (e.g. KT-4004) and it should be very easy to get a 4 bay USB 3.0 enclosure that provide RAID 5 support for less than $220.
A single IronWolf 6TB can provide about 180MB/s, 4x 6TB RAID 5 technically can provide 18TB storage with >500MB/s sequential speed. Maintain 400MB/s should not be a problem.
IMO, this solution provide decent speed, capacity, and redundancy within that budget.
However, if all you want is just some high speed storage for the active project. No need high capacity. I also recommend you to consider 2TB SSD on an internal SATA 3 card. RAID or not RAID doesn't really matter. A single 860 Evo 2TB SSD can provide 500MB/s anyway. 2TB usually good enough for normal size project. As long as you only keep the active project on it. 2TB should not be too limiting.