The "problem" here is how does a collector approach this area.
It seems that you look down on the gold stamps of the Khmer Republic. Check eBay, see what the price they fetch now.
Although Cambodia and Laos rarely give you items over a thousand USD, but investment can come in quantity shots, and this is just what the Chinese folks doing. Sitting in the auction rooms are the minorities, the majorities are buying tens or hundreds of the same items and resell when the price is right.
Two years ago I bought a certain overprint at 5USD per in block of 25. I resold them at 50USD per. This is an investment, isn't it?
Stop thinking about the Chinese red monkey and "the whole country is red". Get something reachable.
Maybe I'm a bit of a stuck-in-the-mud about what I collect. I find the Royalist-era Laos & Cambodia issues attractive for their fine designs and printing. Even the early 70s multi-colour issues still maintained a high standard (such as the Art commems). The Khmer gold issues just scream "collector inspired tat", they obviously weren't issued with postal use in mind, that's why I think they "have a bad smell" to them.
True though, they exist in limited quantities, and are of popular thematics (Olympics, UPU, trains), so there will be a market for them. If one wishes to add them to their collection, I won't look down on the person for it. What one chooses to add to their album is a personal decision.
The 'emergency' or 're-establishment era' overprints for this area are great items--limited quantities, quite a few inverted ovpts or errors in the date to add 'spice' to a collection. The reservation I have though is, given that they're just simple type-set dates, wouldn't they be easy to forge?
Buying Chinese stamps--the Monkey stamp is often described as a "barometer" of the market. Its price shoots up, it must mean the China market is strong. The price droops, it means buyers are getting cautious. Obviously too much of a generalization. On the stock market the basket of blue-chips used to measure market movement has 25, 50, 100 stocks. For China there's so many sub-areas--Large Dragons, Red Revenues, Gold/Silver Yuan, Cultural Revolution, 1970s-80s material. A price spike/drop could be caused by a number of collectors coincidentally shifting focus at the same time into/away from one of those.
As for the All Country is Red stamp, or the 'Treasures of the Republic' rarities, I wouldn't invest my money in those (assuming I had the 5-6 USD figure amount they sell for). Those are "trophy" items. The first rule of investing is always "diversify", so putting so much into one item, an item for which the collector base will always be thin, I'd say unwise.
Go for the moderate to medium-value items, $100-1000. Something that is worth $1 now will probably never rise much in value, as its probably sitting there at $1 because there's tonnes of them around. Scooping up ovpts at $5 and selling at $50, that's a small coup, but how often do such opportunities come along?