Robyn, i would go with 5 SoS lvl 1 in your highlands, then just keep unlocking lands and building colony lvl 2 until your 80b land or so. At this point go back and swap those SoS for vol lvl 3, and make each colony up to that point lvl 3. Then keep exploring and putting down col lvl 3, until land price is 150b or higher. At this point make each t5 lvl 4, and then continue the same.
You may want to explore some HF lands before carrying on with HL, if so i suggest you buy the first 9 and place t6 lvl 1 of your choice on all 9 before returning to HL.