Chapter 4 is definitely where the rubber meets the road with regards to Investment complexity, it's far harder to get everything you need from then onwards if you're on a PRoN-light run.
Some General Analysis: Of the three Country Trade Route investments, Tak'kan's is the worst, as it offers no particular special bonuses or discounts. An optimized playthrough will probably want all three of these, but if you have to cut one Tak'kan's the one to do. Doing this means you'll likely need a heavy investment into Tak'kan in Chapter 4 Phase 1 to get good results in the following event, but it is possible in some cases, I wouldn't count on it though. It's also worth noting that these are the only investments in this cycle that cannot be gotten in a later cycle for a higher price.
Min's Trade route is fantastic for the price, from a money, stat and extra bonus perspective. Probably the least skippable.
Eustrin Guild is huge money (pays for itself in two cycles), and provides a nice bonus on a somewhat tough check much later in the game. If you don't get this you'll be even more under the gun on cash later on. It's 100k more to buy after this opportunity and you'll miss out on some if it's big payouts.
Yhillin Succubi Trade is also very good value for the price, but it's primarily a Big Money investment without any major side benefits. Still very worth squeezing in. Has I believe the highest total ROI of any investment in this set if bought with the Discount.
Gasm Falls Trade is a weird one. It would be nice to get, but most of my optimized runs that I've concepted out just can't fit it in at this point. It's worth noting that this one has a relatively low "discount" for buying now, only 50k, rather than the 100k for a lot of the more expensive ones, so there's less of an opportunity cost for skipping this at this point.
If you sided with Orilise, Denmiel Archives has poor returns, but great stat boosts and is a gift that keeps on giving over and over in terms of helping out with various events. You're definitely going to want it at some point if not right now.
Mushroom Farm is mostly a "Filler" investment that you grab with any cash you have left over. Definitely worth it if you can squeeze it in though, actually totals up to a shocking amount of cash over time.