Many construction companies do the same as you do, where the quantities are worked out and their "unit rates" are used to calculate a cost, in some estimating tool or excel. Then some use MSP is just as a schedule of tasks without any attempt to assign resources or costs, and others go as far as you by including the resource assignments.
The problem with this approach is that the bill of quantities only gets you to the quantities and costs (of those quantities) but not yet a schedule.
I think this traditional process is putting the cart before the horse. If you start with the plan in MSP, you can build all of the resources and cost assignments there, and bypass the other tool and spreadsheets entirely.
I have nothing against import maps and VBA but I think you may be looking for an elaborate solution, when your "problem" can be solved more easily. It can't be that big of a project. How many lines of tasks do you have? A few thousand? Nothing.
If you are assigning resources in the task information dialog, you're doing it the slow way. There are various tricks to do it faster. Use the assign resources dialog. Use the details split screen. Use the resource names column and lots of copy/paste and fill down. Use filtering, sorting and grouping to make the data input easier.