Don’t add the full pre-compiled Bootstrap. Have a look in my repo for how I do it. I add Bootstrap via Node package, then load only the parts that I need and work excessively with the recently introduced CSS variables of Bootstrap.
If you use the linked module then run hugo mod get -u ./... in your project root to update to the latest Bootstrap. Yes, use 5.2, it’s much better than 5.0 (CSS variables were introduced in 5.2).
PS: CSS creation is handled by my hugo-head module, see here.
With the precompiled CSS, you’re getting everything in Bootstrap, which adds a ton of weight to your site.
The Hugo module or Node package makes it easier to add Bootstrap’s SASS files and only enable the Bootstrap features you need. I use the Hugo module on my site because I didn’t want to add another package manager (NPM) just for a styling framework.