10.13 to Big Sur - HDD APFS System volume and HFS+ User Space I want to install either 10.15 or Big Sur on a small APFS partiton on an internal 1 TB HDD in a Mac Mini. I've clean installed OSX and many other OSses before and am quite comfortable installing OSes.
I'm on an older OS X version and some updated applications I use are now 10.15 or higher.  There's only one OSX install on the drive and I don't want to dual-boot the system.
Since one APFS partition is now mandatory for the System Partition, I would like to meet the mandatory requirement. I don't see why the rest of the system cannot have one or more discrete HFS+ partitions for user data and applications.

Late 2014 Mac Mini, I've replaced the OEM HDD with another HDD with spinny platters and size of 1 TB.
The main reason I'm avoiding APFS is there's reports of high fragmentation on traditional HDDs. SSD fragmentation isn’t an issue like HDD and my concern is eventual slow down.
There's also been reports of major bugs, loss of utilities Apple and 3rd party.
I played around with 10.14 and 10.15 with one APFS only partition and didn't like it. I'd tolerate it only as one smaller system-only APFS partition.
Also APFS Copy-on-Write issues are not clearly documented

I don't think this was adequately tested and other OSes still allow older file systems to be used on install. Its beyond me HFS+ would be removed from the installer outright.
HDD Volume
Partition1  &gt; OSX System Volume  &gt; 60 GB (I think Big Sur installer says 20 GB is minimum)
Parttition2 &gt;  HFS+
Partition 3 &gt;   HFS+
Would the above Partition layout work?
No, what you describe is not at all workable by my definition. I think you can go all in with APFS and use an attached external HDD and Time Machine to limit your performance impact. If at such time you eventually have a slow system, you can reset that fragmentation by an erase install. This assumes no utility will get upgraded to help you with optimization, you’ll still have an out with Apple only tools.

Big Sur makes design assumptions that the user data is on APFS so you may regret forcing it to accommodate HFS+ storage both in the steps needed to set it up and the performance of the system in the end.
If you don’t trust APFS, don’t run Big Sur except for in virtualization as a guest OS is my recommendation - stick with older OS and tools you trust.
Detailing all the “bugs” and debunking them would make a wonderful masters thesis to set this up and document how it performs, but even as a hobby, I couldn’t imagine learning enough from this to make it worth the hassle. But, that’s just my take - I might be totally wrong on your skill level and benefits from attempting this. There are wizards here that do things with partitions and non-standards setups / hard partitions that make my hair stand on end with delight and awe. You might be one of those awesome people or connect with them here.
TLDR; what you propose is technically possible - go forth and do it and report back. The definition of “workable” is flexible enough for experiments like this to be worth doing if you are motivated enough.
All pigs can fly, most people are unwilling to supply enough thrust.