Just to give you more ammunition: the Diazo example I gave you on the other thread is part of a Plonified bootstrap 4 theme, which we use on our Plone 5.2 site. We chose to do this because at that time no other alternative was available in 2019 when we started conceptualizing the front end. It was a struggle to fit square pegs in round holes, and still feels unfinished. I would not do this again.
Three years later, the improvements in Plone 6 (classic) with respect to bootstrap and ES6 support make implementation of a large custom theme much easier and straightforward. A huge amount of work went into simplifying things: Plone 6 Classic - weekly sprint