Fusion reactor is still available as a 4th tech in the last era and it bothers me.

So I'd suggest one of the three following path to remedy that which to me is an issue:

The first solution would be to make some other technologies requirements to get to the fusion reactor. Here are some options:

1. Having Power Lines and/or Civil Engineering required for Nuclear Fission » to carry what will be producted by the Nuclear Plants, that as big projects could do with some Civil Engineering
2. Having Computing and/or Automation required for Fusion Reactor » Not sure any would actually be required, they seemed the best suited techs from a gameplay/reality perspective but I might be completely off.

Alternatively, a new tech could also be created between Nuclear Fission and Fusion Reactor and the Nuclear Plant attributed there, with maybe Power Lines and/or Civil Engineering as requirement(s). The submarines would stay at their place.

A visual might help:

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To the bold mind, there's also a more radical approach, involving a change of perspective in the tech tree. Here's how I see it.

1. Keep the fiver last technologies (Exo Suit, Space Orbital, Military Laser, Neural Implant & Fusion Reactor) in the Era VI technically speaking but give them the status of »future techs«
2. They should cost a lot more to research but each tech from the same era that would be discovered would reduce the cost of those techs. The idea is that the more you're coming closer to future techs by discovering advanced ones, the less alien those techs should be to your researchers.
   1. I'd give them for example a 300% increase, with each of the 25 other era VI tech giving a 3% discount, making them effectively the same price if you researched all other techs before.
   2. The science affinity could get a bonus here if you're in the era VI (no French shenanigans without transcendence)

Voilà!
Thanks for bearing with me until the end.

Agreed. I’ve never rushed Fusion Reactor but it’s always seemed weird how it’s done. Pushing it to behind some other tech(s) later in the tree is definitely a simple, solvable way to do this.  That said, your “Future Tech” idea is pretty neat too. I like it. 

I always,  always rush there. 4 techs to get 50% production boost. It Wins the game for me every time.

Especially when you can rush with Science culture in Industrial era. As French you can invent Fusion Reactor before you invent Steam Engine.