Use of privilege escalation in a secure environment with become/ansible - ansible

Question

I want to perform administrative tasks with ansible in a secure environment:
On the server :
root is not activated
we connect throught ssh to a not sudoer account (public/private key, I usually use ssh-agent not to type the passphrase each and every time)
change to a user which belongs to sudo group
then we perform administrative tasks
Here is the command I execute :
ansible-playbook install_update.yaml -K
the playbook :
---
- hosts: server
tasks:
- name: install
apt:
name: python-apt
state: latest
- name: update
become: yes
become_user: admin_account
become_method: su
apt:
name: "*"
state: latest
The hosts file :
[server]
192.168.1.50 ansible_user=ssh_account
But this doesn't allow me to do the tasks: for this particular playbook, It raises this error :
fatal: [192.168.1.50]: FAILED! => {"changed": false, "msg": "'/usr/bin/apt-get upgrade --with-new-pkgs ' failed: E: Could not open lock file /var/lib/dpkg/lock-frontend - open (13: Permission denied)\nE: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), are you root?\n", "rc": 100, "stdout": "", "stdout_lines": []}
which gives the idea that there is a privilege issue...
I would be really glad if someone had an idea !!
Best regards
PS: I have added to sudoers file the nopasswd for this admin account and if I run this playbook it works :
---
- hosts: pi
tasks:
- name: install
apt:
name: python-apt
state: latest
- name: update
become: yes
become_method: su
become_user: rasp_admin
shell: bash -c "sudo apt update"
I guess that when I changed user via su command from ssh_account, I would like to specify that with the admin_accound, my commands have to be run with sudo, but I failed finding the right way to do it...any ideas ??
PS: a workarround is to download a shell file et execute it with ansible but I find it is not satisfying...any other idea ?