Questions about Ubuntu 18.04 LTS Install / partition sizing on SSD / HDD system

Unallocated SSD space for provisioning

Addressing this part of the question:

Plans are to shrink my Windows ntfs partitions down to 450 GB, and
expand the Ubuntu ext4 partitions up to 500 GB, leaving 50GB
unallocated. I read that it is helpful not to allocate SSD’s fully as
it improves performance / error correction /stability.

From: SSD: keep unallocated space?:

Second, most SSDs are overprovisioned internally. Your 256GB SSD
likely has something like 256+16GB of actual space in it. The SSD
controller hides this from the OS, but it will internally use that
extra room when needed.

SSDs can write to free, erased space faster than rewriting existing
data. As long as the SSD and OS have TRIM support (all do these days),
deleted data will be “erased” and future writes will be fast.

Therefore I wouldn’t keep 50GB unallocated. I would allocate it to Ubuntu and increase 450 GB to 500 GB.

Separate /home

I would simply keep 100 GB / and 350GB /home together to lessen chance of either one running out. Similarly I would never have a separate /boot partition.

I keep my /swap in 8 GB SSD partition instead of HDD so it will be faster but it is never used so if ever wanted the space back I could relocate it to nearly empty HDD.

Plans for HDD is to store large windows projects (60 GB). Then in the weeks/months I’m working on a given project it is moved into SSD. After project is over it’s moved back to HDD and a different project is moved from HDD to SSD.