You're right that 4 colossus is too much. 2 or 3 should be good.
I don't think the manticore is a good idea here. You'd better use
fling to sac colossus and have more
ravenous intruder(I would play 4 of them in this deck) and padeem instead.
Key to the city: should be 4 of as well. It is a key card for the win.
Instead of
Metalspinner's Puzzleknot you should use
implement of examination: overall it cost less mana without the life lost drawback.
All those cards can help you get colossus:
Reason // believeStrategic planningLifecrafter's BestiaryGlassblower's PuzzleknotHowever, if you intend to cast colossus by using mana, maybe your best bet is 4* Inventor's fair. Then you do not need to put in your deck so much cards to search for colossus. This option also helps for a 2 colossus build as inventor's fair kind of garantee you to get it.
More spire industry won't hurt : you only need to pay life when needed but at least you are sure to get the mana color you want.
A deck like this can not go out without some underhand design: it will put some pressure on your opponent, get you some life back and it's a additionnal creature removal.
another lead to
explore is
refurbish for some graveyard mimic to cast colossus.
Other artifacts/cards to consider:
-
foundry inspector-
Chief of the foundry -
Contraband kingpin-
pia's revolution-
Saheeli Rai: attack with 2 colossus ?
Edit: the sideboard looks fine and covers a wide range of threat for the different matchups. I believe
efficient construction should be mainboard. it's perfect in this deck to get you flying blocker for free or artifact to sacrifice to
ravenous intruderHope it helps.