At the time of this writing, IonAccordionGroup does not provide a method that lets you do it in the way that you described.

Your “workaround” is the correct way to do it.

You can create a custom Angular directive to reduce complexity:

import { ContentChildren, Directive, QueryList } from "@angular/core";
import { IonAccordion, IonAccordionGroup } from "@ionic/angular/standalone";

@Directive({
  selector: "[appAccordionGroupToggle]",
  exportAs: "appAccordionGroupToggle",
  standalone: true,
})
export class AccordionGroupToggleDirective {
  // get a reference to the ion-accordion instances inside
  @ContentChildren(IonAccordion) accordions!: QueryList<IonAccordion>;

  // get a reference to the ion-accordion-group instance
  constructor(private host: IonAccordionGroup) {}

  expand() {
    // setting the value of the ion-accordion-group to an array of all the 
    // ion-accordion values will expand all ion-accordions
    this.host.value = this.accordions.map((accordion) => accordion.value);
  }

  collapse() {
    // setting the value of the ion-accordion-group to an empty array will 
    // collapse all ion-accordions inside
    this.host.value = [];
  }
}

In your component, you can use it like this:

@Component({
  ...
  standalone: true,
  imports: [
    ...
    AccordionGroupToggleDirective,
  ],
})
export class MyComponent {
}

<button (click)="toggler.expand()">Expand all</button>
<button (click)="toggler.collapse()">Collapse all</button>

<ion-accordion-group appAccordionGroupToggle
    #toggler="appAccordionGroupToggle">
  <ion-accordion>
    <ion-item slot="header">
      <ion-label>First Accordion</ion-label>
    </ion-item>
    <div slot="content">First accordion content</div>
  </ion-accordion>
  <ion-accordion>
    <ion-item slot="header">
      <ion-label>Second Accordion</ion-label>
    </ion-item>
    <div slot="content">Second accordion content</div>
  </ion-accordion>
</ion-accordion-group>