Hey @y-nara,
it all depends on what you’re looking for.
Static HTML & CSS can be a good approach, but from a marketers/editor perspective it might be not the right one.
Something I ask every company I’m start working with is: do you want to have more flexibility or more “compliance”. Depending on this will be my setup. For instance if the company tells me “we want 110% design compliance” it means that editors will be restricted to content editing in the custom modules and might only have an option to choose the premade design from. If the company tells me “we want full flexibility” it means that editors will have full control over everything, but it may also result in results that are not design-compliant. Your page, your desicion. 
From my experience static HTML & CSS templates are great if you don’t want to provide to many options to editors, be more on the compliant side of things but this has the drawback that if you need to change something, you will have to apply the changes in the template and then those changes will be applied to all pages that are using this template. Can be benefitial but also some sort of drawback since you might end in a situation with several, almost identical templates which will be quite hard to handle in the long run.
If you’re looking for more flexibility, I’d recommend to create a theme with drag&drop templates(dnd_templates). Those allow your editors to edit pages individually while still using the same template. Furthermore you can set up whole “reusable” sections for common used content/layout. Or you can create “row-modules” with dedicated layouts and functions(those are also possible in static HTML templates)
Note: Once you’ve create a page with a dnd_template and change something(space, text, image…) HubSpot will automatically create a dedicated and not accessible version of the template. This is good since future updates to the theme won’t affect existing pages.
As for custom modules - endless possibilities here So congrats on the choice to use them. Can you tell me, what you want to create?
Hope this helps.
best,
Anton