Honestly, if we're talking about finding good colossus x reader stuff, Archive of Our Own is basically my whole life these days. The tagging system is absurdly specific, so you can filter for exactly the kind of dynamic you want—protective, established relationship, angst, fluff, whatever. I've found some incredibly tender fics there that treat the whole 'gentle giant' thing with real nuance, not just as a gimmick.
That said, the Tumblr scene for this pairing is weirdly active? It's more fragmented, but there's a lot of shorter pieces, headcanons, and drabbles that pop up if you follow the right tags. The search is garbage, but once you're in the network, people reblog each other's work constantly. I stumbled onto this one series that was all about the reader helping Colossus adjust to modern life after being away, and it was so charmingly written.
Wattpad can be a hit or miss, but sometimes the algorithm throws something decent your way if you've been reading a lot of X-Men adjacent stuff. The quality varies wildly though, and you have to wade through a lot of... let's call it less polished work. But I did find one author there who writes these slow-burn office AU scenarios that are oddly compelling.