Hi,
I have HP DL380e Gen8 I think the same that it is impossible to upgrade it with LFF.
My advice is to upgrade the HD drives. Now it costs only 100 Euro for a 4TB HD drive.
That is what I want to do for my DL 380 Server. By the way, it is used so it is very worthy: https://www.servershop24.de/en/server/hp/dl-series/dl380/gen8/