Need proactive talent sourcing techniques that build pipeline before roles open

This is specifically directed at people without large TA teams. Struggling to keep candidates because we're running into the same pattern of role opening, sourcing from zero, being slow, and losing candidates to faster processes. The problem's evident but not sure how to practically build a proactive pipeline when the TA team doesn't have sourcing capacity sitting idle between searches. How do you stay warm with candidates who aren't relevant right now without it becoming a full time relationship management job?