Lets get a few basics out of the way for going to space.
1) Tech path: You want assembly line early for factories + coal plants, also Infantry. Then beeline Computers to build the Internet. Usually I try to trade for techs on the bottom of the tree (especially Combustion line, sometimes Radio line).
2) Infrastructure: You want to have some cities to produce spaceship parts. In total there are 16 parts, so there is no sense in preparing more than 16 production cities. Production cities will need Factories + Power and Labs, health buildings onyl if required and after production buildings. Try not to starve cities.
3) Civics: For production Caste + SP have great synergy. Unhappiness from emancipation can be mittigated by trading techs away to switch vassals out of emancipation. Production cities should have workshops + watermills all over the place. Keeping towns is OK for research, non-outdated resources need to be modernized, but the rest should be replaced with watermills + workshops. After (or shortly before) research is done towns can be replaced. Btw US is OK since you have democracy, but unless you have the kremlin buying anything is rather inefficient, even with the kremlin it is arguably unseless in this case, since it competes with research.
Now about your current situation, I have a very important question: Did Charly start plotting before or after HC vassaled? This is relevant, because the AI attitude is the average of the attitde towards the master and all vassals, in this case, since he is friendly with HC he is effectively pleased and still can not plot.
Charlies current stack has artillery, cavs and rifles. Unless he gets tanks that can be dealt with with cavalry (attacking a stack outside a city is much easier than taking a city). You can get assembly line in about 6 turns, after that with factories and infantries, a little later tanks you become untouchable. If no one is plotting against you you should not be building units but wealth or infrastructure.
Regarding Machine guns, IMHO these are amongst the weakest units in the game due to not being able to attack. If someone attacks with siege you do not want to sit there and take it but counter-attack. MGs do not help to do this.
Also HC has a few cities left. Trade techs with him and tell him to research stuff that helps you (Astronomy, RR, Combustion, ...). Also he can give you some resources you do not have, especially health ones.
But why isn't it mechanical or dull to go space?
It is, although differently than conquest, it is more of a building game than a wargame, which I prefer. In the end it is a matter of taste.
We are for the initial situation, we just do not arrive at the same conclusion. Since this is really only a minor argument, I suggest to leave it at that. In the end the important part we disagree about is the width of the usefullness of espionage, and in the end I think this also is a matter of taste.