Does anyone know how to make a free-form landing page responsive? We have a desktop and mobile versions, but I was hoping to add more breakpoints (tablet, small desktop, etc.). Any advice will be greatly appreciated.
This is not possible, period.
You will have to build or get new Guided templates, then rebuild all your landing pages with these new templates and finally replae the old with the new LP everywhere they are used (smart campaign, smart lists, ...).
Do not forget to also create redirections so that all the inbound links to your old LPs are redirected to the new ones.