Etrecheck Report, Could you please help?

I didn't see any of the most common culprits that cause sluggish behavior.

I'd try booting into Safe Mode and see if it's any better in that mode. If it runs better in Safe Mode, this indicates there's a third party extension or app causing the issue. Safe Mode does clear out some caches and does some storage cleanup, and it sometimes cures certain ailments. See this Apple Support article: Start up your Mac in safe mode - Apple Support

The only thing that caught my eye was that Browser Company app was mentioned more than once. Might try a different browser and see if that helps.

Perhaps someone else reading this will spot something I missed.

"On the cloud" is great for sharing photos, but is not a viable backup solution for everything you have. The stuff is not under your control, and is subject to sloppy handling, arbitrary changes in policy, theft, accidental deletion, data loss [are they making frequent backups using best practices?], and discontinuation or throttling of the service. It can easily take three days to restore it at ordinary Internet speeds.

If you do not have a recent local, disk-based backup, your computer is like a ticking Time bomb. You are only one disk failure, one crazy software, or one "oops" away from losing EVERYTHING! Drives do not last forever. It is not a question of IF it will fail, only WHEN it will fail. In addition, you never know when crazy software or Pilot Error throws away far more than you intended.

If you are using another direct-to-disk backup method that you prefer, and you currently have a recent disk-based backup, that is great. If not, you should consider using Built-in Time Machine. Take steps to acquire an external drive as soon as possible. If you buy one, a drive 2 to 3 times or larger than your boot drive is preferable for long term trouble-free operation. Do not pay extra for a drive that is fast.  (You can get by for a while with a "found" smaller drive if necessary, but it will eventually become annoying).

Attach your external drive and use

System preferences > Time machine ...

... to turn on Time Machine and specify what drive to store your Backups on.  It may ask to initialize the new drive, and that is as expected. APFS format is default format if running MacOS 11 Big Sur or later.

Time machine works quietly and automatically in the background, without interrupting your regular work, and only saves the incremental changes (after the first full backup). Time machine backs up every connected drive that is in a Mac compatible format. it can not back up Windows format drives.

Time Machine's "claim to fame" is that it is the backup that gets done. It does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there.

How to use Time Machine to Backup or Restore your Mac:

Use Time Machine to back up - Apple Support

MacBook Pro 15 2015 i7 16GB often got hot and made noise, tried cleaning it, updating thermal paste, resetting the SMC, reinstalling the OS, disabling animation, switching to the old Catalina OS, cleaning startup... there was 0 effect.

Here's what helped:

1. Install the free application "Turbo Boost Switcher" and click on the "Disable Turbo Boost" function.
2. Install the free application "Macs Fan Control" and configure both fans in it - "Sensor-based" in a temperature mode of 60°C and 90°C. The first fan was installed on the CPU Proximity, the second on the Intel Iris Pro GPU (or whichever one you have).

After this, the fan noise disappeared. The temperature is not rising. Productivity has not decreased. I ran high-load tests with these parameters and they all passed successfully.

It feels like I bought a new Mac.

you are being punished for choices you have made:

Backup:

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   Share services:

        Add to Things - /Applications/Things3.app

        OneNote - /Applications/Microsoft OneNote.app

        Twitter - /Applications/Twitter.app

      Spark - /Applications/Spark.app

 Finder sync extensions:

        Backup and Sync from Google - /Applications/Backup and Sync.app

        Core Sync Helper - /Applications/Utilities/Adobe Sync/CoreSync/Core Sync.app

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This means you spend WAY too much time determining what files need to be synced, and too much online time syncing them.

Re: MalwareBytes: Users here have great respect for its comprehensive list of Virus strings. Scanning with MalwareBytes, ONCE, is a fine idea. However, running it at all times was never good idea, and your copy has crashed twice. Turn that runtime scanning OFF.

<<. I come from Windows with multiple hdds/sdds >>

The Mac supports as many external drives as there are places to connect them, and then some. Time Machine backs up every Mac formatted drive connected to your computer, automatically.

I don't see any know Bad Actors, and nothing is hogging CPU or Memory. Your network numbers suggest there still may be Apps "phoning home" a lot more than expected.

Your RAM memory is only 8 GB, but it has not overflowed onto the Swap file on Disk. Your drive is a bit slow compared to the latest Macs, but faster than many. It appears to have TRIM enabled and is not full.

Is this report typical of an overheating time? if not, I suggest you run an Etrecheck when your system is overheating and let's take a look at that one.