Rabbi Eliezer’s view seems very plausible to me (though it was not accepted in the Talmud): “The Musaf prayer is only with a town congregation.” The problem is that my reasons are rather tainted by “it’s just too long,” so I wanted to hear the Rabbi’s opinion as well, since you’ve said several times that in your view there is room to omit parts of the prayer that are not completely “obligatory.”

Too long is not a reason. By the same logic you could simply say you just don’t feel like it. And especially with an opinion that was rejected in the Talmud itself, one should not act in accordance with it.

That is indeed the meaning, I just didn’t want to write it so bluntly…
On the other hand, “I don’t have the strength” is also a reason — as Maimonides rules following the Talmud in Eruvin 65 (not specifically related to Musaf), that if a person finds his mind confused, “it is forbidden(!) for him to pray until his mind settles.” And who exactly concentrates and directs his heart when counting the piles of animals in Musaf? The very thought already makes “his heart preoccupied”… So it may be that someone who “doesn’t have the strength” is actually forbidden to pray Musaf at all, and one who does pray is uttering blessings in vain.

*And that is without even talking about the alienation and antagonism that the content of the prayer arouses in some worshippers, especially vegans and vegetarians. But in any case, the question is in general and not necessarily specifically about vegans.