MacOS Sequoia 15.0.1 fill up free space on the Macintosh HD

Yes, see this...

Install macOS on an external storage device and use it as a startup disk - Apple Support

Start up from macOS Recovery

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Look at top of screen for Utilities menu...

In Disk Utility>View, select Show all Devices, highlight the top left entry.

Run Disk First Aid on all items in the left panel, from top down.

Did you try Disk 1st Aid in Restore mode?

Then try a safe Boot & then We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info. Give it Full Disk Access.

https://www.etrecheck.com/

Give etrecheck Full Disk Access before running.

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

Yes, immediately get 2 external drives, install the OS on one of them, then try to copy what you can to the second one, maybe even use Carbon Copy Cloner seems to accomplish cloning it to the second driveit...

https://bombich.com

Yes, immediately get 2 external drives, install the OS on one of them, then try to copy what you can to the second one, maybe even use Carbon Copy Cloner seems to accomplish cloning it to the second drive...

https://bombich.com

If you've been running Time Machine regularly launch Disk Utility and see how many local snapshots you have on your boot drive. You can delete them by selecting them and using the "-" button at the bottom. Always leave the latest two for safety sake:

Boot from the external drive and run Migration Assistant to import all your apps, Docs, Settings & other files.

• Navigating to Terminal:
• Once in Recovery mode, go to the menu bar and select "Utilities" > "Terminal". 
• Using Terminal commands:
• Once in Terminal, you can use standard Unix commands to manage files, disks, and perform other system operations within the recovery environment. 

cd Users/bd1/Documents/Parallels/

Spaces asre very important in Terminal.

ls -a

Do you see Windows 10.pvm listed? Next is the dangerous command...

rm Windows 10.pvm

You could only remove data from Macintosh HD - Data, Macintosh HD is closed off for the Systems use, if the problem is indeed there, then erasing the whole drive would be required.

What are the biggest files on Macintosh HD - Data.?

Thanks for the report. There are a couple of issues that I see;

1 - your read and write speed is below average :

Write speed: 935 MB/s

Read speed: 966 MB/s

You can run the free version of DriveDX and see what the status of your boot drive is.

2 - you have some external drives connected that could be contributing to the performance issue. Disconnect them and test for a bit.

3 - you have some unnecessary software installed: EnigmaSoft. There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu)

4 - you have some software, qemu-system-x86_64, that is using up quite a bit of system resources, CPU cycles, memory, etc. Don't know what it is but if not needed you can remove to and test.

5 - Android Studios is crashing frequently and using memory. Is it up to date?

6 - lastly is Parallels. ±It crashed 93 time in the last 60 days. Make sure it's up to date.

That is a Virtual Machine of a Windows® 10 Install, used by Parallels to run Windows® on your Mac.

If you no longer need or want it...

https://www.youtube.com/watch?v=Se2PPB1ufDI