Have an early defender’s advantage that diminishes as the game progresses. You could for example make powerful base defenses that are cost effective against early units, but are weak against upgraded or higher tier units like artillery and aircraft. Tech progression is often on a different timeframe compared to regular production, that way players can’t just spend more money to unlock tech faster. While each player is waiting for the offensively useful techs, they’ll want to spend their remaining money on base building and defensive units.
Tie the economy to base building. Normally, you have to expand to more vulnerable locations to increase your income, but you could make it so resources are mainly found near your original base and/or make it so that your initial base can be significantly upgraded (adding on new modules to your refineries or something, maybe even get income from the total “population” of your base) to increase your income. This would encourage building a single large base and make it harder to harass the other player’s economy early on, but you’d still have to have some change in pace later on (like superweapons or the higher tier offensive units above) to make sure both players eventually have an incentive to attack.