To maximize manpower just grow your cities and realm. Tier 2 buildings from all categories less so for Production and Commerce has Manpower attached to them. Non-mixed pops (your ethnicity) provide the biggest bang for your buck, so can plan Conquest accordingly. Now to the Metal issue. Relevant buildings all in the Infrastructure. You want regions with natural metal veins where the most common are iron and tin. You should edict for Roads and Ports only. Always go for the infrastructure in the new regions first.
You can blitz through building queues by using this methodology:1) Every region should have 3 tier 1 infrastructure buildings to unlock free roads, free builder hall, free 2 levels of land expansion, potential mines if you get lucky on Prospect Resources.
2) Supporting buildings for highest value are Military, Stewardship and Health in that order. Largerly ignore food (unless percentage based or food region) and Piety. Use Fortification Military buildigns as your shuffle option to not waste Authority on Edicting. Build up Piety support to uphold 3-4 per POP standard.
3) Supporting buildings is fine since they are pseudoshuffle as well, but decide your region priority. Coastal and/or Central Domain/Province region fit for the Commerce buildings, plains for food, Rough for infrastructure. Thats your 3 basic region types, where food onr has the least priority. Provincial Capital and your border regions for Military. And in case if Capital you want to maximize stewardship + piety for Education. Remaining for stewardship especially if they are far away from the capital.
For trading: Just go on buildings spree whatever produces the most resources once you figured out which region focus you want. It will pan out. Once growth curve slows down around 15 POP in most regions with decent starting POP open trade window and reevaluate (shift+2 & T when region selected). You can build structures with 3xResources cost if you knkw that those are pretty common and will pop up later down the road rather soon. As far as RGDs go: maximize Forge Claim, all recruit decisions, all population promotion, all vassal related, spy & gift to nobility and develop region. Everything else can be dumped in grey zone until occassion arrises which can promt the signal boost temporarily. For Army Compositions: you want your SAUs to be infantry and cavalry. Cant waste these slots on medium infantry or skirmishers. Those unit types handled by Levies and Mercenaries. As long as you have army wide enough to cover Rough frontage which is 2 flanking squares for cavalry and 8 squares for the infantry/cavalry + 6-8 supports you are fine. Try to avoid Deserts and Plains meanwhile. Due to how proximity to the center mechanics work your best bet is to have 4 cavalry, 6 heavy infantry (for ranged defense and assaults early to midgame) or 4 heavy/2 medium backed up by 8 archers. For the open terrains and prolonged sieges pad out the line with more medium infantry. Have like 4 heavy infantry, 4 medium infantry, 8 cavalry. Usually the idea behind more cavalry and mediums besides lower upkeep is flexibility. You can fight in all types of terrain and intercept by virtue of being fast after splitting away from heavies who gonna keep up the siege going. Try to fish with edicts for Siege Workshop to get the early artillery spawned. If you can afford it have your 2nd stsck composed from mediums, cavalry and supports for mobility and killing smaller nations. It really depends on how your opponent forces compositions looks like. Sometimes you need to brute force with all heavies.
Remember to use Retaliate often to train your troops. Destroy their mobile stacks sk they cant get behind you and then split to mop up remaining regions by straight up assaults (0 or lvl 1 walls) or sieges. Can assault lvl 2 or above if their only defense is garrisons and you have 2 exp stars heavy infantry to cover the frontage and 2 offense general. For prolonged besieging want 1-2 general and mostly medium infantry army. 0-2 or 0-1 riskier if opponwnt sorties. Game doesnt always recognizes them as attacker in that scenario. For naval you actually want 1-1 general due to how impulses systems choses randomly who the attacker most of the time since you are not directly targeting opponent fleet in 90% of cases. You want 14-16 ships, most of them are light. Have like 3 havies and 3 mediums. Cant split easily since Naval AI is more competent than in Empires. But still can use 1 light ship to blockade ports once their main naval forces been dealt with.
A bit on Conquest RGDs:
On Court Expenses and mini-tips for Byzantine: