The IValidateObject is indeed a good option. I did not realize that the validations could be implemented in a flexible way. Below an example , Text1 is now fixed and Text2 is dependent on the Status.

@using System.ComponentModel.DataAnnotations;

<Select @bind-SelectedValue=mydata.Status>
    <SelectItem>New</SelectItem>
    <SelectItem>Completed</SelectItem>
</Select>


<Validations @ref=valref1 Model=mydata Mode="ValidationMode.Manual">
    <Validation>
        <TextEdit @bind-Text=mydata.Text1></TextEdit>
    </Validation>

    <Validation>
        <TextEdit @bind-Text=mydata.Text2></TextEdit>
    </Validation>
</Validations>


<Button Background="Background.Primary" @onclick=Submit>Submit</Button>

@code {
    someData mydata=new();
    Validations valref1;

    async Task Submit()
    {
        valref1.ClearAll();
        await valref1.ValidateAll();
    }

    class someData : IValidatableObject
    {
        [Required]
        public string Text1 { get; set; }

        public string Text2 { get; set; }
        public string Status { get; set; } = "New";

        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        {
            if (Status == "Completed")
            {
                if (string.IsNullOrEmpty(Text2))
                {
                    yield return new ValidationResult("Text2 is required", new[] { nameof(Text2) });
                }
            }
        }
    }
}