Create custom sector lookup for new leads in Salesforce

We need to allocate new leads to an industry sector to route them to the correct account manager. Leads are created in capture formats that don't suit the enquirer filling in their own sector. We have a lookup table of 15,000 company names with our bespoke sector assigned, and currently download all data, do a bulk assignment in Excel, and re-upload, but this is time consuming and inefficient. The lookup list is also too long to code into the lead assignment rules, so we want to explore if a developer can create a custom application that will allow Salesforce to look up the sector in the table when new leads are created, and before the lead assignment rule runs. (The lead assignment rule uses e.g. some selected key account names, sector, and alpha-splits to allocate leads.) This solution should be easy to update so we can change the list or the assignments as and when we require by uploading a new list. It is desirable for the solution to also be applicable to other lookups, so for example, if we create a jobtitle / seniority list to apply the same process.