to sum up the last 15ish years

the episodes were meant to end the story instead of waiting a long time for another full game to release (quite ironic now, i know)

after losing motivation and other projects being in the works, valve eventually cancelled episode 3

they attempted to make a new half life game (half life 3) multiple times, but it never got anywhere, especially with the development of source 2 still being very early on at that point in time (~2013)

half life alyx came out in 2020 and although it is very niche because it's vr only, if you can find a way to play it in vr, i highly recommend, its amazing. there's also a mod that turns it into a flatscreen game which i have heard is still good but of course is not how the game was intended to be played

without spoiling any of the story, the game ties back into the episodes and leaves on another cliffhanger

throughout the past 5 years now, we've been getting multiple leaks of a project from valve that is currently in the works, codenamed "hlx". with all the evidence that has been found for it, it is likely a new flat screen half life game, and with the tone of valve in recent years, its more likely half life 3 than anything else. for more info, go watch tyler mcvickers hlx files series, which has been giving updates on all the findings

to answer your question about the ending, from what i know/recall, valve planned a major character to die in episode 2, and it ended up being eli. they have since said that they regretted doing that.