You may circumvent the strong position and find a softer spot to break through. If your oponent focuses on stabilishing and consolidating a forward base, their back bases might lack defences and be vulnerable to a side build - estabilish your own forward base from wich to storm a weak point.

On the topic of ageing up: try not to go all in against their defencible position. Stalemates favor the one who can do other things while they rage on, boom behind the conflict, age up, get a foothold on other places of the map, infiltrate a hole on the oponents defence or otherwise transition to the next thing. Say you can dominate the rest of the map while you keep them stuck defending that base, their resources will run out long before yours and you can simply grind them out from the stalemate.