I would distinguish two main directions. It really depends on your products and skills of your team.
Somebody on your team is a frontend architect, meaning that she/he can implement anything from a visual design producing well structured and clean code. She/he could even reimplement any main features of Bootstrap in reasonable time without any frameworks.
In this case your team should focus on building a deep vanilla CSS/Less/Sass skillset and build the components, layouts and design from scratch.
Your team will be able to build unique components with high quality but will not be efficient when needs to deliver similar websites fast.
Bootstrap, Pure CSS, Material-UI etc.
Your team knows well Bootstrap or any other frameworks, has a pretty good frontend stack skillset and can rapidly customize it to achieve the vast majority of customer requirements.
In this case your team should focus on getting to know with several CSS frameworks and pick one, in which the team specializes itself.
Your team will deliver nice websites really fast but struggle when requirements became more special.
It is always a good question whether to dive deeper or to learn more frameworks. Depends on your products, customers and your team. It is always a good idea to try both directions and specialize in one based on real experience.