Report Server data refresh options

PBIRS runs SSAS behind the scenes with a built in mechanism for scheduling data refreshes. Unless you need control over your tabular models or have other constraints (see below), then I'd drop your plan to host SSAS tabular and just use what PBIRS gives you with your reports running in "Import" mode.

Contrary to my advice, we have actually implemented our own SSAS Tabular instance in a similar architecture to what you've described. Because our finances are limited and we can only afford PBIRS under an SQL EE with SA licence for 4 cores only, we came to the conclusion that the server running PBIRS/SQLEE would not be fast enough to handle our usage requirements, so we implemented SSAS under our SQL Standard licence on an extremely fast server.

If money were no object, I would not have done this. I would of used PBIRS out of the box.