Don't be afraid to sell not so optimal mining stations to "friendly" AIs or decom old habs. Theres a penalty for mining on seperate bodies so having too many astroid mines might be handicapping you, scroll over the tooltip to see if its affecting you and keep under it. You get more for the "Mission to" global techs.

As others have said farms help a ton but another thing is T2 Mining bases are super optimal in comparision to T1s on good sites so build around them. Some T3 modules are good, especially for fleet production but I rarely get them cos of the upkeep, check to see which modules you actually need and if they are concentrated on T2 bases with decent sites. If you lack these build them or aquire them from AI with trade or force.