10.13 to Big Sur - HDD APFS System volume and HFS+ User Space
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.