I would recommend that you look at a medium-term strategy to move all software and services into the cloud and find alternatives where this is not possible. In previous guises I have transitioned schools to be entirely cloud-based. This has many, many advantages. No need for expensive on-prem servers, with the need for maintenance and backups. No need to rely upon VPN tech, or other half fangled alternative approaches. No restrictions on what Operating Systems are used, or even device type in many scenarios. If a device fails, or is lost, the data is not lost and at lesser risk and the user can carry on working using an alternative device. I could go on and on...
...Actually one more thing, not really any need for a local directory service to manage either.