Searchpartyd using 100% CPU on intel iMac

You already accomplished a significant amount of troubleshooting to no apparent effect. The only suggestion I have that is even remotely applicable would be to sign out of iCloud and sign in again. It's the iCloud equivalent of "turn it off and on again".

Personally I think you may be going off into the weeds with Bluetooth. It was good idea to suspect something amiss with it, but searchpartyd is not limited to BT. It's part of Find My as you wrote but that service is all-encompassing. Given the actions you already took I would tend to rule it out as a contributing factor.

They said the next trick would be to start in safe mode and then to, possibly, reinstall the OS. However, this behavior on the intel Imac has persisted across a number of OS upgrades so I'm skeptical that a reinstall will actually do anything.

So am I, but lacking any other obvious causes do it anyway. Try Safe Mode, test, reboot normally, and if the problem persists throughout that experiment proceed with reinstalling macOS. Like you I doubt it will help, but do it to placate Apple's request.

There are quite a few system modifications on that Mac, many of which I would advise against using, but I doubt any of them are related to the problem at hand.

I tried, but failed, to get the apple chat person to admit they knew this was an issue.

It's certainly not widespread. Scattered reports, but that's it. Apple denies problems much bigger than that 😃

Turning off iCloud and turning it back on again dislodged something. What that was may never be known.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate towww.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

I understand the EtreCheck report is difficult to read and that following Old Toad's User Tip instructions would be most helpful.

However the following is immediately apparent:

Security updates disabled - Security updates are disabled. This computer is at risk of malware

infection.

Apple security disabled - Apple security software is disabled. This computer is at risk of

malware infection.

I suggest the first order of business is to rectify that deficiency immediately. There is literally no disadvantage to leaving them to their default settings, which allow Apple to install critical time-sensitive updates required to keep macOS secure from emergent threats.

Software Update settings on Mac - Apple Support

Do that first. Then and only then turn your attention to posting a revised EtreCheck report.

On the intel iMac, once searchpartyd starts running it uses 100% of one cpu and never goes below that number.

For what it's worth the subject of your concern does not appear to be a factor in that report. There could be other causes that might be more visible if you were to capture an EtreCheck report when it becomes evident. Then, post it according to those instructions.

Also I shall ask the site Hosts to redact or obscure the personal information shown in your previous post. I can't guarantee they will do that or even know what I'm asking of them, but don't be alarmed if you see [edited by Moderator] or words to that effect.

Great! Glad to hear it.