Is there a good diagnostic type program along the lines of SiSoft Sandra for Mac ?

NO.

All of the aftermarket stuff like that is scare-ware, whether it is competent at doing what it claims or not.

for day-to-day casual insight into what is happening on your Mac, I suggest:

• do not install such stuff, at all, ever,

• Use activity monitor to see how you Mac is performing

Activity Monitor User Guide for Mac - Apple Support

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if you have SPECIFIC complaints, rather than using junk like Sandra, they are many good aftermarket utilities for actual SPECIFIC problem solving. Many of them are low-cost or free.

EDIT: I see while I was slowly typing, my colleagues have suggested the EXCELLENT low-cost/free discovery Utility Etrecheck. I can not possibly say enough GOOD things about Etrecheck.

Etrecheck was developed by senior contributor here, and uses system calls to collect often-needed information.

Etrecheck contains little tests for speeds of devices, CPU utilization, memory usage, energy usage and a digest of recent problems, in one easy to use package. it does not even need to be Installed. Because less can be learned when your Mac is running great, best time to run is when your problems are actually occurring, if possible.

if you follow the directions faithfully, its report (pre-laundered of all personally-identifiable information) can be "Shared" to the system ClipBoard, then Pasted into an ‘Additional Text’ window in a reply on the forums.

How to use Etrecheck Pro for free

http://etrecheck.com

...

What exactly are you trying to find out? If you're having a particular problem, it would help people to make recommendations if you said what it was.

The first place I generally start is etrecheck:

https://www.etrecheck.com/en/index.html

It is highly trustworthy. The developer is a long-time forum member.

2pods2 wrote:

Thanks everyone.

Not having a problem at the moment, I'm just nosey

I think I'll be purchasing EtreCheckPro.

The other valuable thing about EtreCheck is that it was designed to enable you to post a report here in the forums that gives all of the relevant system data but no personal information. If you do have a problem in the future, it will be useful in getting help here.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

For help deciphering the Report, see > How to use the Add Text Feature When Post… - Apple Community

2pods2 wrote:

Thanks everyone.

You're welcome.

Not having a problem at the moment, I'm just nosey

Me too, I run EtreCheck Pro every couple of months to compare Performance and Diagnostics.

I think I'll be purchasing EtreCheckPro.

Access to the power package, is totally up to you.