Firstly, don't use BitTorrent to get Apple updates. The risk that they have been compromised is too high. You should always get updates from Apple.
Before Big Sur, Apple provided updates for download at Support Downloads. So, download once and update one or more Macs was possible.
With Big Sur, Apple stopped providing downloadable updates. You now have this choice (which you may not like):
Let the Mac download and install the update (2 to 3 GB) on each Mac - as you have been doing. And, yes, you may need to start again if there are any problems.
Download the complete macOS (12.5 GB). Save a copy in case of install problems before using the complete macOS install to update you Mac.
Where to get macOS?
MrMacintosh has a page dedicated to the downloads of complete macOS for Big Sur. Note carefully that the links he provides are all to Apple servers.
The page also includes sha256 digests for each download. As well as a way to check the accuracy of the download, it would also be a way of checking the accuracy if you do choose to download from elsewhere.
MrMacintosh has other useful pages. How to download describes various ways you can download from Apple's servers - many of these are scripts or apps to assist the process. For myself, I use the MDS method.
From the question, I think you have just one Mac to update. If you had multiple Macs there is also Content Caching.
As you have probably realised, the size and frequency of updates has increased with Big Sur. Read Squeezing a thimbleful of update into a ten-gallon can for a thoughtful diatribe on this issue.
Edit:
For the Big Sur 11.5.2 a checksum on the macOS install application from Apple can be calculated using:
cd .../Install macOS Big Sur.app
find CONTENTS -type f -exec shasum {} \; | sort -k 2 | shasum
d7bcbae11844285c60a3d871710381e4e8acde73
[The find and shasum command was taken from Stack Overflow answer]
For macOS install app from another source, the checksum can be calculated in the same way and compared.
As an experiment I found a non-Apple torrent file. The downloaded install app had the same checksum.
But I am not going to use it!
For torrenting to be a viable and safe method, you need a trusted friend with a reliable connection to download the install app from Apple and compute the checksum. And, who will repeat this for every macOS update.
