We had an implementation partner. BW was implemented  after SAP Dealer Business Management. The vendor had necessary level of expertise.

We also require time for maintenance. Currently, we have a monthly and weekly schedule.  Maintenance activities like Change log deletion, deletion of Persistent Staging area tables and compression of requests in cubes are automated. Periodically we get requests for enhancements or new report development . We also periodically do some  fine-tuning of aggregates  to improve report performance. 

Normally we do not use a vendor or integrator to implement this solution. We only use a third-party if it is requested.

We implemented through a vendor team. The vendor's had the necessary expertise.

First we used SAP, then moved to a SAP partner.