Can you work around it by massively overprovision by partitioning the drive and leaving half of it unallocated? The amount of space you need for an older system like this should be tiny compared to modern storage.
That works as long as you prepare the drive on a machine that does support TRIM, to ensure the unpartitioned area gets TRIMed one last time before the drive is moved to the old machine. Then it should remain in that state as long as it's never written to.
Even if you didn’t do that, I wouldn’t expect the partitioning to write to the unallocated space. If you start with a fresh drive I’d think it should work.
Yes if you trim it after making that partition and system correctly informs SSD about empty space. Secure Erase before making partition would be the safest bet - that way SSD firmware has full control over free unallocated space.