This is a really big question. To start of, as you know, a whole lot of things changed between 1000 and 1400-1500 in Europe, too much to list. In 1000 Byzantine Empire was still a major actor in european politics, the Islamic states ruled most of the Spain and Sicily as Normans hadn't yet descend into Mediterrenean and South Italy to turn the tide while William hadn't conquered England. The Crusades hadn't kicked off yet. Religious wise Great schism was brewing but didn't happen yet, and only a few monastic orders existed. In trade and maritime things the Commercial revolution of the 12th century hadn't happened yet, compass wasn't known, and first trade ships didn't began regularly passing strait of Gibraltar until very late 13th century. Add other technical innovations the gunpowder, the printing press, the improvements in mining and metallurgy, blast furnace, windmills and watermills, and the list goes on and on. You can write a book on each of these changes and how they affected everything.

So I'll took the cheap way and focus only on one of the things you mentioned.

First:

I'm aware that the desire to find a trade route to Asia emerged because the Ottomans' 1453 capture of Constantinople cut off the land route

I am actually insufferable in reiterating over and over again how Fall of Constantinople hadn't closed the land routes to Asia and how it had little to do with Iberian start of discoveries. I think for your question it's important to be aware of that.

To go the second thing I think it is most directly involved with your question:

the Portuguese had already sort of set up "practice" colonies in the Canaries and the Azores, making them "ready"

Oh, but the Portuguese had so much more beyond just there "practice" colonies. They have started exploring and raiding/trading down the West African coast since 1430s. By 1450s they had already reached Senegambia and Sierra Leone, and by 1492 they had already passed the Cape of Good Hope.

During this multi-decade long endeavor they weren't just sailing by the coast, but they had also by intent and by effect established an economically sustainable system based on trade/exploitation of the new found regions. The system was based on large scale acquiring of slaves, gold, ivory and other valuable items of the region in exchange for european goods of what we consider lesser value. To secure this trade, and then later to maximize their profits, they through much trial and error came to conclusions how 'best' to organize themselves. Which turned out to involve creating permanent settlements as well as in-discriminatory usage of force and violence, enabled by their advanced gunpowder weapons - pretty much godfathering in the practices of colonialism.

But I want to stress out that this was a slow, incremental thing through the stages. And reading first hand sources of Portuguese in the early era of their expansion (1400-1450) you can actually see this as very much continuation of the medieval and crusading/Reconquista mindset rather than more modern thoughts of colonialism. Basically we aren't talking about the joint stock venture companies nor use the justification of the exploitation through some kind of racial genetical supremacy ideas you might find in 19th century philosophy. But we are talking kings going for more land against their neighbor or noblemen leading raids like they did locally, or small merchantmen going for a small voyage to trade horses for slaves. And the justification fir violence and slavery was heavily accented as either a typical medieval war, or through the religious angle (Christians vs. non-christians). At some point the idea to trade goods with Asia emerged as main motivating factor and goal, but again this was a typical medieval thing to think about as Venetians and others had been getting rich on Asian goods for centuries (and as mentioned in linked thread not stopped at all after 1453)

So when we are talking about Columbus expedition in 1492, we aren't talking about something happening in a vacuum or for the first time, we are talking about a another - but unorthodox - manifestation of expeditions Iberians were conduction for more then half a century. Which is why it shouldn't surprise us why it didn't surprise people. Although it could be also argued it was surprise in more ways than one. It took some time for Europeans to come to terms to what finding Americas meant. Columbus himself, and few others after him were still looking for way to Asia long time afterwards. Venetians were unconcerned with Spanish findings, but news of Da Gama and Cabral reaching India freaked some of them out. Outside Iberia and Italy, lack of interest and giving importance to Americas was even more pronounced. Cabot's discovery wasn't really followed up by the English, until Elizabeth times. The Dutch/Flemish were also satisfied with being the destination and prime resale hub of the colonial goods, and only after independence from Spain in late 16th century did they take up exploration itself.

What my conclusion of this long winded rambling would be, is that by Columbus time Iberians have for some time been involved in exploring and exploiting and developed a mindset and a system reflecting that. For your question on 'why did these become issues of importance' I wouldn't say they necessarily became of importance out of the sudden, it's just that pursuing this avenue - which was a typical continuation of the medieval expansionist policies - turned out to be very profitable and sustainable so it became self-perpetuating