Liberals will point to how improvements in quality of life have occurred in capitalist countries in recent centuries (debatable, and certainly not true for the entire world, but let’s assume they are correct for now).

This isn’t debateable. The Imperial Core states and its outposts through the world have living standards more luxurious than ever seen before. We also shouldn’t place the Imperial Periphery, on whose falling standards of living the living standards in the core depend, to the side–this exploitation formed a key part of Marx’s analysis. Improvements in quality of life in the Imperial Core go with and are predicated on increasing impoverishment of the periphery.

famines were a harsh reality of life for much of human history, modern agriculture has allowed us to now be in a position where globally, we can produce more than enough food consistently for the whole planet.

This is only true if we refer to history in the narrowest sense (i.e. only the written past) and famine in the broadest sense (i.e. any period of going hungry is famine, regardless of whether or not there were deaths).

Famines in the broader sense are endemic to history in this narrow sense. When you have a stratified class society with surpluses and urban populations doing writing (i.e. fulfil the narrow sense of history) you have an (urban) population with control over distribution of the grain. This population, historically, will restrict the consumption of the ‘lower classes’ to maintain its own (often already inflated to include luxuries as ‘necessary’) consumption, which often leads to relative hunger (but not always or even usually deaths).

As you mention Chinese famines later on, here is an excerpt from Late Victorian Holocausts describing the Qing government’s response to the famine relief in 1743-44, not paralleled in contemporary European famines (where the starving peasants died for lack of cash for the crops they sold to pay rent):

the renowned “ever-normal granaries” in each county immediately began to issue rations (without any labor test) to peasants in the officially designated disaster counties.8…When local supplies proved insufficient, Guancheng shifted millet and rice from the great store of tributegrain at Tongcang at the terminus of the Grand Canal, then used the Canal to move vast quantities of rice from the south. Two million peasants were maintained for eight months, until the return of the monsoon made agriculture again possible.

Famines with certain deaths are an invention of the last few millenia, and are particularly concentrated at first in Europe, and thereafter to greater, more rapid extents, in places it colonises. In the narrow historical sense, “producing more than enough food consistently” has been the norm. In the broader sense of history, i.e. the entire human past recorded in writing or not, famines seem to be even less of a regular occurrence. Archaeological evidence suggests that, contrary to this stereotype of hungry and starving hunter-gathers turning into regularly fed farmers, the hunter-gatherers were consistently fed from a variety of sources with less wear and tear from labour.

Likewise in regards to medicine… in the past just getting sick could be a death sentence. People had to live with incredibly painful conditions their whole life that we now have cures for.

Not really the case (outside of the (ofc horrible) childhood mortality rates). In a lotta cases, medicine went backwards with the enlightenment as remedies/coping methods/etc for pains were regarded as “superstitions” that didn’t “truly cure the [incurable even today] ailment”. And ofc throughout the world (including Europe at some point) cultures that live on the land tend to know a variety of local medicines based on herbs, animals, plants, etc. These methods are ofc sought after today by multinational corporations so they can copyright them and sell them.

Honestly modern medicine is the one reason why I would rather live in 2023 than any other time.

When you say this are you accounting for the fact that, if you were randomly born in 2023 there is a >80% chance you’re born in the Imperial Periphery and therefore, rather than accessing such medicines you’re the one suffering for their production? You might be downwind of some industrial ‘development’ that gives you and your entire family diseases no one has ever heard of before, or you might have your entire village burnt to the ground to mine the minerals or harvest the plants used for the medicines up in the global north. Heck, even in the US or Canada we are still settler-colonial entities; you could very well be born up somewhere in Northern Alberta where much of the food you eat is contaminated with oil and the water has various industrial runoffs.

What I’m getting at is… though these advances did occur under capitalism, I don’t think I would give capitalism the “credit” for them.

I would give imperialism the credit for them. Having a large population that doesn’t have to live with the negative consequences of its luxuries (i.e. a nobility) is a key part of imperialism (and a necessity for surplus value realisation).

Obviously socialism was not possible 200 years ago. I’m not denying standard Marxist historical progression.

Did something fundamentally change between 1823 and 1848 when Marx began saying “socialism is possible”?

Marx doesn’t have a “standard historical progression”. Marx has a method of analysing society and looking for ways to transform it for the better. In his early works (e.g. the manifesto or german ideology articles) there is a schematic tendency, but this is abandoned mostly in Capital and totally in later notes and letters (e.g. to Zasulich in 1881). Marx’s near uncritical positivity towards capitalist development of productive forces also fades very quickly, to be replaced with scathing critiques of machines, agriculture (animal and plant), colonisation and peasant-clearing.

I think the only things you can give capitalism “credit” for is developing the productive forces, allowing for high levels of commodity production, and increasing levels of wealth (though not equally shared).

Because of how critical he was before the great acceleration and expansion of consumerism, I would think that Marx’s view on the current ‘development of productive forces’ would be rather more critical than you’d expect, especially since his views on all this don’t even take greenhouse gasses into account so he’d probably be even more negative on this.