Late 2015 iMac continues to freeze after hardware replacement, testing, wipe/restore, and user file migration

Issue

iMac freezes between 1.5 and 5 hours after being booted. No clear cause or consistent software active.

Computer

• Late 2015 21.5" iMac 4K, 2TB Fusion drive, 16GB RAM, Intel graphics 1.5GB

Background / History

• 14 months ago, hard drive was replaced by Apple after similar issue and no clear cause
• 3 months ago, logic board was replaced by Apple after similar issue and no clear cause
• Last week, hard drive was erased, 10.14.6 installed from recovery boot, system restored via Time Machine (figured block-for-block restore might've been the problem)
• Earlier this week, hardware and software were stress-tested for two days at Apple Service Center, then hard drive was erased, 10.14.6 installed from recovery boot, user files restored via Migration Assistant after secondary admin account created

Attempts to Fix

• Ran EtreCheck Pro, no major issues found (no antivirus apps or major disk utilities installed, no rogue or outdated daemons)
• Erased Fusion drive, reformatted, reinstalled OS 10.14.6, migrated user files
• Reinstalled combo update 10.14.6 from Apple.com
• PRAM reset x3, SMC reset x1
• Ran disk First Aid on hard drive via recovery-booted Disk Utility

Current Status

1. Turned off Spotlight indexing across all local files and external drives (USB 3.0 connections x4)
2. Tried Time Machine backup overnight with screensaver on, came back to frozen screensaver
3. Rebooted, saw multiple "mdworker" and "VXPTC-something" (related to iPhoto video encoding) messages, hangs, and logs in Console.app
4. Turned off iCloud Photo Library in System Preferences > iCloud > Photos (first checkbox) based on research into point 3
5. Tried Time Machine backup again (last seen "Removing old backups" then "Backing up..." this morning)

I'm at a loss. What can I possibly do at this point? It's obviously a software issue, but how do I fix it? I fear that even if I did wipe the drive, reinstall 10.14.6, and manually migrate my user files, the issue would continue to occur. The issue may have been helped by turning iCloud Photos off, and I'm continuing to check my Plex Server's uptime this morning... TBD if any of the above helped.

I'm more than happy to post any EtreCheck or Mac OS system logs as required, I just want my computer to stop freezing! Thanks for any help anyone can provide.