I only ever really had to start over after Third Arclent War, seeing just how useless Carina's men and the Mercenary Flotilla were.
After that, there is a pretty clear optimal route. Sacrifice Carina's men, postpone Yhilini Bank, Sanitation Mages, and Mercenary Flotilla to Chapter 4, and get Denmiel Archives and Mercenary Offices in the war aftermath.
For Chapter 4 first round, get Theltiar rentals (Social +1), and then go ham on investment returns. Get Airship Fleet, and Cee'kan Shipping on second round (and anything else for Tak'kan 40). Also get HQ military 20, Magic 10, and Hospitality 7+. Before Synod, get Lustlord Expansion, Givini Tunnels, Stineford Expansion, and Elleani 90+ (Teahouse > Smithing). Try to get Tak'kan mine immediately after Synod. Open ruins before war, get HQ military 25 and Magic 30, get EVERYTHING in Kyangan (Smithing cost increase is too high to justify getting any other profitable investment available at this point). Then wrap up last round by getting Democracy, Succubus band tour, and Sanitation Mages (not available afterwards), Gasm Falls Water project (Tertia RP), maybe Airship Militarization (for better Patrici bonus next round), and finally all profitable investments (I only had to skip Succubus Armourer).
During this, remember to keep Givini score in mind for Givini Orc Merchant returns. I sent artifacts to New Givini, favoured them in Magic standards, and also bought decoration set before last round, so I just hit 50 on last round. Only 50k more overall, but, as of 0.68.0, that will allow me to just get everything in the next round of investments.
Chapter 5 is way more relaxed. In round 1, get all profitable investments on first round, orc enhancement investments for Second Gathering, and Feroholm Restoration. For round 2, rush to hit Rodak 50 to get best price for Trade investment. I skip Zirantia Infrastructure and choose to enforce order during Incubus War. Still, even if you want to get Infrastructure, buying Trade at 200k now is the same as buying it at 100k in the next round, so decide whether that 200k is better spent elsewhere or not. Remember to hit 12+ Ivalan Acceptance, if possible. You have a lot of leeway in that we just don't have enough information to make razor's edge decisions. Same goes for Round 3.
Givini monument... is largely relegated to last round. It just isn't useful. Never was. I doubt this would even have been a point of contention if not for Thosha's playthrough and Fulminato's ardent support. Too bad people aren't as enthusiastic about 100k OoS, or early Denmiel, which are both gifts that just keep on giving.