までだ

1. 1If other options fail, one will simply do this, stated with resolute calm rather than distress.Ex. 1
2. 2With the た-form, one only did this and nothing more, often to downplay one's own action.

Sense 1 usually follows a conditional clause such as 〜なら, 〜ば, or 〜ても. It announces a fallback plan in a composed, somewhat stiff tone, as in "If he doesn't come, we'll just do it ourselves." Sense 2 explains or downplays one's own action, as in "I only did what I was asked," and unlike 〜しかない it sounds matter-of-fact rather than resigned.

• しかないN3しかない is close only to dictionary-form までだ; it stresses that objectively there is no other choice, with a nuance of reluctance or resolve, and it can also attach to nouns meaning 'only.' までだ is crisper and more resolute, usually follows conditional clauses, and has the たまでだ usage that しかない lacks.
• よりほかないN2よりほかない is close only to dictionary-form までだ, but it emphasizes having no other means and being forced, with a written/formal flavor; it can also attach to the ない-form. It does not take the ta-form and has no 'merely... that's all' meaning.
• 〜ほかないN3〜ほかない is close in meaning only to dictionary-form までだ, both meaning 'have no choice but,' but it conveys reluctantly accepting the only way out, without the decisive, ready-for-anything tone, and is more formal. It has no たまでだ 'merely... that's all' usage.
• までもないN1までもない means 'there's no need to go so far as to...,' judging the action to be unnecessary; までだ means 'worst case, I'll just...' or, as たまでだ, 'merely... that's all.' までもない attaches only to the dictionary form, so the two cannot be interchanged.
• 〜にすぎないN2〜にすぎない is close only to たまでだ: the latter attaches only to the ta-form of verbs and mostly explains that one's own action has no other intention; the former can attach to nouns, verbs, and adjectives, coolly limiting the scope of things, and is more formal. It has no dictionary-form usage expressing resolve.
• ざるを得ないN2ざるを得ない attaches to the ない-form minus ない, is formal, and stresses having no choice due to circumstances or logic, usually with reluctance; dictionary-form までだ, by contrast, sounds crisp and resolute. It has no たまでだ 'merely' meaning.

しかない, よりほかない, and 〜ほかない stress that there is no other choice, often with resignation, and ざるを得ない adds a sense of being forced against one's will, whereas までだ conveys a calm, almost nonchalant resolve to take the fallback option. までもない means "there is no need to," and 〜にすぎない means "nothing more than" and mostly follows nouns, whereas た-form までだ downplays the motive behind one's own action.