To run a full node and lightning node, I always read that people should use SSDs. Is that really required? And why is that? It's a slowly growing blockchain after all.
I would have everything for an umbrel node, except a big enough spare SSD.
A SSD is not required for running Bitcoin, but it certainly helps. You would definitely want one if you were running a production service using Bitcoin, but for something you're using personally it will just make synchronization substantially slower to use a hard drive. For a Raspberry Pi you absolutely do not want to be storing any data on the Micro SD card it uses as a boot disk, as this will result in corruption at some point.
It's not required but recommanded. The main (only?) difference is that the first synchronization of the blockchain will take much longer (unless you download it separately, but it's not the native way of doing it with Umbrel: https://umbrelinfo.gitlab.io/faq.html#can-i-use-previously-downloaded-blockchain-to-save-time-when-setting-up-my-umbrel-node ).
To give you an idea of the order of magnitude, with an SSD that would take about 3-4 days, with a HDD it's more like 3-4 weeks.
It depends on how patient (or skillful) you are in the end :)