Have you considered something along the lines of coercive portal to help get you to the combo quicker or is serum powder sufficient?

For reference this is the list I ran last time I was on combo shops, obviously quite a different build to yours and it's about a year old now

4 shop
4 tomb
4 buried ruin
Academy
2 city of traitors
7 solomox
1 Mox opal
Crypt
Vault
3 grim monolith
4 voltaic key
4 metal worker
4 Serum powder
4 Leyline
4 forge master
2 lightning greeves

Time vault
Helm
Blightsteel
Staff of domination
Sundering titan
Trinisphere
Memory jar
2 coercive portal
Spine of ish shah

SB
4 mistep
4 cage
4 mountain
3 smelt