In wandering around the Barcelona Turisme website to answer a question on this forum, I stumbled on tours of two modernistata buildings I hadn't see advertised before. I think they haven't been open to the public very long. Each has just one English tour per week, so one would need to plan ahead. Obviously, I haven't been inside either of these places, but the photos look nice:
Palau Baró de Quadras, Avda Diagonal, 373. Designed by Puig i Cadafalch. 45-minute tours in English on Wednesdays at 11 AM; 10€. https://www.llull.cat/monografics/palaubaroquadras/english/index.cfm
Casa Manuel Felip, Carrer Ausiàs March, 20. Designed by Telm Fernández Janot. One-hour tours in English on Fridays at 11 AM. Website in Spanish: https://irbarcelona.com/edificios-de-barcelona/casa-manuel-felip/. Barcelona Turisme website in English: https://bcnshop.barcelonaturisme.com/shopv3/en/product/24494/casa-felip.html?o=h