Here is an easy to read guide on what to build as the Borg to maximize your economic efficiency.
I also recommend Playing on Grand Admiral difficulty as this increases the likelihood that the AI will keep colonizing systems which are a benefit for the Borg.

I made this guide due to upgrade limits on assimilated planets and primarily placing buildings that don't upgrade (at least above tier 2) on assimilated planets. One big distinction is that it's more efficient to build Mining(Production Nodes) on Unicomplex's and build Energy(Nodes) on Assimilated Planets. Assimilated Planets do not have any buildings that increase mining efficiency. Which it's possible this is a bug, as it shows that there should be a building that can be built on planets when you research the upgrade to mining buildings.

Nanite(Analysis Nodes) Are inefficient on Unicomplex's vs Orbital Nanite Foundry buildings as they can upgrade on Unicomplex's.

You don't have to follow this guide strictly at least until about the mid-game. Since by then you will have a good mix of Unicomplex's and Assimilated Planets.

Try to keep your Unicomplex to Assimilated Planet ratio 1:1 until the late game where you run out of Unicomplexes/Subcomplexes to build. At which point your assimilated planets will outnumber the number of Unicomplex/Subcomplex's as you assimilate the galaxy.

A good rule of thumb is to build more unicomplex's/assimilate planets as you run out of room for your population. As well as planet scooping in order fill in your empty/partially filled unicomplex's/assimilated planets. As for planet scooping prioritize high populated worlds, preferably far from your borders, as you want to use systems close to your main borders for them to be turned into Assimilated Planets. Only upgrade starbases on systems that have unicomplexes, subcomplexes, and assimilated planets. As well as prioritize getting systems that have raw dilithium. And assimilate a planet in that system or put a subcomplex in that system with an upgraded starbase.

ST:NH
BORG Economic Guide

Unicomplex build:
1. Housing(Drone Hives)
2. Maintenance Stations (I-II)
3. Mining(Production Nodes)
4. Stratified Astro-Mining Complex (I-II-III)
5. Orbital Nanite Foundry (I-II-III)
6. Research(Long Term Data Archives (I-II-III)
7. Hive Nodes (I-II-III)
8. Synethetic Dilithium Manufactuary I-IV)
9. Drone Refitting Chain (I-III)
10. Alloy Forges (I-III)

Assimilated Planets build:
1. Housing(Drone Hives)
2. Maintenance Stations (I-II)
3. Energy(Energy Nodes)
4. Power Hub (I-II) (power nodes can be upgraded to III but it's only a difference of 40%-45%)
5. Dilithium Refinery
6. Hive Defense Nodes (I-II)
7. Neutron Enricher (if you are short)
8. Nanites(Analysis District)
| ONLY if Unicomplex's run out of room for Orbital Nanite Foundry's
& want to make room for more Research(Data Archives) OR Alloy Forges on Unicomplex's

The order of Traditions to take (all according to what I personally think is best) is:
1. Productivity
2. Adaptability
3. Versatility
4. Assimilation
5. Perfection
6. Synchronicity
7. Resilience

Ascension Perk Order: (Again what I personally use. Plus I use a mod that adds 20 ascension perks which are researchable and seems to be compatible with New Horizons)
- 20 Ascension Perks / Slots - mod

1. Fleet Maneuvers
2. Galactic Force Projection
3. Imperial Perogative
4. Technological Ascendancy
5. Mind Over Matter
6. The Flesh is Weak
7. Grasp the Void
After this choose what you think is best to you and doesn't matter too much the order since you will be researching new ascension perks if using the 20 ascension perks mod.