@EscoMP , thanks for your post, and for sharing your experience.
You are correct - as noted on the Theme documentation page, themes in Studio only apply to colors, font style, and borders. You can override individual components with the Style panel options int he Properties panel for each chart, as you have noted.
I really think that your use case can help us improve the experience. If you are able, would you be willing to submit your feedback to our team? You can do so by following the directions here: https://support.google.com/looker-studio/answer/7340016
Thank you again for sharing, and for your ideas for improving Studio!