You’ve already found your synonym (all at once), and the OED has your back. It even uses in one go to define it:
all, adj., pron., and n., adv., and conj.
a. all at once.
(a) With everything happening in one go or simultaneously; at one and the same time; all together.
Source: Oxford English
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Here are some selected usage examples given:
1588 W. KEMPE Educ. Children sig. F2v A sillable
of eight letters, being too hard for a childe to learne all at once,
he may learne letter by letter.
1662 R. MATHEWS Unlearned Alchymist (new ed.) §82. 109
She..popt it into her mouth, and swallowed it all at
1706 Phillips’s New World of Words (new ed.) at Orgues
Several Musket-Barrels set in a row within one wooden Stock, to
be discharged either all at once, or separately.
1930 Lancet 27 Sept. 686/2 The recently prepared
solution is warmed to body heat and may be quite safely injected all
at once provided it is injected slowly.
1995 Daily Mirror 23 Feb. 6/4 Transfer of other
powers to the new Assembly likely to be phased in rather than
introduced all at once.
I wanted to crush the pepper all at once, so I used a bigger tool.