@Thomas Brown good questions, and the fact that you're thinking about this before you build rather than after is exactly the right approach.
The MultiNav Pro+ is self-contained in the sense that the amplification and receiver sensitivity optimization is handled on the module itself — the external PA and LNA stages are part of the RF design, not something you need to add on your carrier board. That's actually a big part of what separates it from commodity BLE modules where you're essentially working with the bare SoC output. So no, you shouldn't need to add a PA stage or do matching network work on your PCB side.
On the antenna question — you're right that a chip antenna isn't the move here. The module has a U.FL connector, so you can run coax to whatever external antenna makes sense for your enclosure. For a ground-level sensor node at 5-8km over open terrain, a simple omnidirectional whip (half-wave or 5/8 wave) will serve you well and is easy to position vertically for maximum horizontal radiation. If your base station is in a fixed location and you want to squeeze more link margin out of the link, a small directional antenna on the base station side while keeping omni on the node side is a solid approach — you get the gain where it matters without complicating the remote node.
For your use case specifically — 5-8km, open terrain, fixed node reporting to a base — you're well inside the ground-to-ground spec even with modest antennas, so as long as you keep the antenna clear of the enclosure body and oriented vertically, you should be solid. The main thing people run into in outdoor deployments is antenna orientation drifting (especially if the node is mounted somewhere that can shift) or the antenna being too close to a metal enclosure wall and getting detuned. Keep a few centimeters of clearance and you'll be fine.
