So, I set up a read-only marketing user profile by only checking "Access Marketing Activities". However, it seems read-only users can't even view email design in the campaign. Should I also give them read-only Design Studio? Or how should I set up so they could view email assets?
Yes, I would grant access to Design Studio and enable the specific accesses you need, most likely just "Access Email (0 of 5)". I believe that will allow that role to view emails not just in Design Studio, but also in marketing activities.