Do you have an alternative to suggest?

For the general user? Not really, I'm just venting :) I have the unsubstantiated, possibly irrational belief that MSO UX ought to be far more polished after having existed for so long. Like you said, most of my frustrations have workarounds, even if they are buried or tedious (though the tedium is part of my contention).

For making slides or posters, someone in school recommended to lay out a poster or each slide completely in ChemDraw (basically a specialized, widely used vector-based WYSIWYG editor for organic chemistry) and paste the lot onto a PPT slide. That works reasonably well and makes everything have a consistent look, especially since most slides of mine contain chemdraws of molecules anyway. ChemDraw does have its own warts and somewhat limited functionality beyond drawing molecules and text. Also, since like 2019 the rendering and UI have gotten so much slower. For posters and basic diagraming, I currently use Inkscape, pasting things as needed. Inkscape also has its quirks, but its interface is so much more powerful and the UI so much more responsive than either ChemDraw or PPT that it is the clear winner for me. Though, it is also not winning any startup races.

For Excel, you're unfortunately correct; there is no suitable WYSIWYG spreadsheet replacement. While I can do essentially all of the typical numerical hacking I do in Excel with (IMO) a better experience in LibreOffice Calc, it falls down when I need to show it to someone else. The charting in Calc is lackluster, and it doesn't play nice with Office file formats. Also, my workplace (though not me personally) makes heavy use of Excel UI + VBA + fortran (!) DLLs for reactor modeling...

LO Writer is probably the closest to Word in usability, but compatibility with Word files is still subpar, mostly to do with styling and formatting. Both are bloated for probably 50% of my use (e.g., writing up informal procedures or meeting minutes). Wordpad (RIP) is nice in that regard.

it used to be a direct menu item but now it is quite buried....

My colleague swears that UI in MSO before Office 2007 was so much better, but I can't comment much on that.