You will not find the time — you have to take it. Work expands to fill the hours you give it, so client work will consume everything unless your own content has a fixed, protected slot. The trick is turning the client work you're already doing into content.
Your own marketing falls last because everything routes through you. The fix is to treat your agency like your most important client — give it a fixed, protected slot on the calendar — and to move work off yourself through systems and delegation, not willpower.
The founder should be the primary face — founder authenticity and credibility convert better than anyone you could put forward, and audiences detect when the conviction isn't real. But don't make yourself the only face: keep your voice central while a team handles execution and other people also build presence.