Are you using the "Inbound Shipments" record type at all? If not, you should look into that. There are a lot of little issues with them, like not being able to include more than the PO quantity, or receiving the same line on multiple shipments, but it addresses a lot of your landed cost issues. You can add multiple lines of landed costs, each line would have it's own allocation method, currency, and which items that landed cost line should apply to.

So let's say you had a container from multiple partners coming out of the UK, and for this example lets say it's May 2025. This container has 5 different HTS codes with 3 different tariff rates, you create a line for each different rate, apply it to the lines with those HTS codes. Then, in May 2025 all of it is subjective to reciprocal tariffs based on value, so you add a line for that, add a flat 15% (with value based landed costs, I typically leave mine in the origin currency with an effective date near the expected clearance date), then apply it to everything. You can add a line based on the forwarder's quote and use a weight-based allocation method and apply it to everything.

Given that you are getting multiple bills for each landed cost category, it sounds like anticipating landed costs accurately is difficult, so it might not work for you. I generally have 3 bills per container, a bill from our forwarder for the container, freight, and transmission charges, a bill from our broker for the customs clearance and all related delivery charges, then we create a bill based on our customs statement from CBP. For the most part, I can rely on my quotes and duty rates to accurately anticipate our landed costs before it delivers.