I feel like I have racing thoughts, so this might be a bit confused and/or forgetful (and I apologise for that), but I will try.
I really don't like much of this.
To me, this feels like removing bits from the game and kind of...dumbing it down? Why do I need to be careful with my units? They're free, I could spam units out at an AI planet far faster than it could reinforce, faster than the Warden Fleet could.
It feels like there'd be almost no permanence anymore. Currently, we cannot lose things like a Golem or a Zenith Power Generator permanently. We cannot lose a Fabricator, we cannot Corrupt Design Backups, we can't destroy a Special Forces Base to weaken the Special Forces income. With this, once I've captured a planet with a Fleet, why keep it? It holds nothing else of value, since most planets don't actually hold more than 1 item. Yes, the AIP Increase from a ZPG being destroyed is something, but that feels so little, because I still have the ZPG.
I don't know how we're meant to lose with this. We can barely be permanently damaged - the only path to defeat is just one big strike that kills the Home Station. If I don't need to really hold planets for more resources, my behaviour at this point would be just castle up around my Homeworld, as it's the only thing that matters anymore. Holding a ZPG isn't worth having to keep a Citadel or Battlestation there anymore, since under the new Energy system, all it'd grant me is the ability to power a Forcefield or something like that at home...or I could keep that Citadel and/or Battlestation at home and not pay the AIP for the ZPG.
Losing a Command Station doesn't really hurt (outside of Economics). A Fleet Wipe barely hurts, and if Salvage gives EXP...I might want that? Fleet Wipe = no cost to me, but the Reprisal gives me a permanent power increase in EXP. (Speaking of EXP, if it's gained entirely by just fighting...then that feels like a return to the "optimal thing is to slowly scout the entire galaxy, bit by bit, as the first thing you do" problem - just sit there on low AIP and let the Stations rank up. Hopefully I'm wrong here).
Back to metal, if it's free and we can spam ships endlessly, then gameplay feels like it's now just "spam ships at planet, get Fleet, spam even more ships at next planet, get Fleet, repeat". Since almost nothing can be permanently lost, what do I have to think about? Sending the correct units so I can win the battle, do some minor tactics to preserve my ships? Why bother, it's work for nothing. It's...now busy work. We'd also lose things like Neinzul type ships being dirt cheap and spammable as their selling point and unique trait.
If metal is free, why do we have personal shields? A big point of those is they recharge for free, or can be sped up for really cheap (but still a cost). If metal is free, then repair is free and shields lose that point. It might still work for non-player factions, but I've not cared about that so far. At this point, it's now just a downside, since it means some units kill you far quicker.
Like Badger said, this is a fairly big buff to the player, and...kinda in a way ruins what I think AI Planets are: Roadblocks. Time is needed for the AI to send waves, to get CPAs, etc. Kind of like in Starward Rogue, the enemies and bosses need time to get their patterns out, they have to survive to do that. If I can spam ships forever through planets, then I don't think they'll really be proper roadblocks anymore, and I could likely win the game in a much shorter span of time. Boost my Fleet so I can have a chance at attacking an AI Homeworld? Meh, send stuff into the grinder, I'll wear it down eventually with no weakness caused in myself. Reprisal doesn't matter, since I bunkered up hard around my Homeworld and hey, I benefit from it? I can just keep doing this and I don't get any temporary weakness or anything, really.
Another behaviour that I'd probably do is just bring along a Citadel or Battlestation, and freely set up turrets all over enemy planets. You can do this currently, but you at least have metal keeping you in check.
Energy change feels like the bit I'm least bugged about, but the point above of the ZPG and Citadel/Battlestation and AIP is still here, and...kind of makes it still less likely in general for me to ever capture something for energy. I get the idea of Metal, not having that old..."Netflix Time" and the like, but I just...feel like it'd change the game into something I would never enjoy playing.
I would point to two things in particular that exhibit something like this: Command and Conquer 4, and Zeratul from Starcraft 2s Co-Op Mode:
C&C 4 has no resources at all, units are built for free up to a generic total popcap like many RTS games have. There's not much use for map control, there's no permanence, almost no delay in your actions, almost nothing to do to the enemy except kill them outright. This game effectively killed the series for over 9 years, with still nothing really new in sight.
Zeratul has gas be automatically harvested, his upgrades are automatically researched, and his units automatically do their abilities. The units also cost so much, you barely have to actually order any to be built, and your supply cap is half that of normal as well. He is so incredibly boring, because you really have been automated out of the game.
That last bit I'm worried about now...truly pointless things like Science Labs being needed to gather it I'm fine with, but without metal...the game just becomes crank the speed up and spam units, which whenever that state is reached for me in a game, instantly dooms that save/run/whatever to boredom.