I don't think it's feasible to use an asset builder. The form will be used by about 260 organisations, and has around 50 fields.
There's over a 100 of them submitted per week, so after a very short period there's going to be lots of data. That's why I'd like a dashboard showing not only results, but some charting as well. I'd love to be able to manipulate the data at some stage too.
An asset builder would actually be able to do all of this, and a lot easier as well, especially with the metadata. You will still end up with the same amount of data if you use a custom form because each form submission is an asset. The downside with the asset builder is the sending of the emails, you would have to do it with a trigger. So send email after asset built.
However, if you must use a custom form, then you could apply the metadata schema to the form submissions folder where the form submissions come in. That way any new ones would automatically inherit the schema.
I'm not sure if you can put default values in the metadata fields as the form respone keywords, would have to try it out. If not, then you would have to have a trigger action for each metadata field you want to populate. So with a trigger, set the event to asset created, set the conditions to only apply to your submissions root node and for form submissions assets. Then the action would be set metadata value, and you would have to have one for each form field you want to capture or search on.
So as you can see, it's harder to use a custom form asset and trigger to set metadata, and easier to just use an asset builder and use a trigger to send the emails (if you even need to send any emails). The asset builder could just create data source assets. And you can still search on these and filter by creation date ranges etc.