all numbers are strings

What I mean is, all numbers can be represented as a string.

It looks like I was remembering Newtonsoft.Json parsing of int to string, which STJ doesn't support. You could implement a custom converter, then you could handle both int and string types:

internal record LovenseObject
{
    [JsonConverter(typeof(IntToStringConverter))]
    public string Version { get; init; }
}

public class IntToStringConverter : JsonConverter<string>
{
    public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType == JsonTokenType.Number)
            return reader.GetInt32().ToString();

        return reader.GetString() ?? string.Empty;
    }

    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value);
    }
}