As director of development (fundraising) for a small museum I understand exactly what your issues are. Gift processing is really important and can turn into a huge mess without a system. However, although this is only a small part of your job, IMHO these are not small, mundane admin tasks.
My first question to you is does your organization have a protocol/workflow established for gift processing? That is, policies and procedures on how cheques are transferred from your desk to the accounting office? If so, that should provide the first framework for your system. Otherwise I think your basic plan is sound. A few thoughts:
For general, routine gift processing: Depending on the volume of gifts coming in and the number of acknowledgements to be written, you might consider separating the two components of the process: 1) entering the gifts into your fundraising database and transferring the checks to accounting and 2) writing/processing/mailing the acknowledgement/thank you letters. In my office, we do 1) DAILY , because getting the gifts to accounting is a priority and 2) WEEKLY. In an ideal world I’d love to send thank you letters out daily, but we are a small shop, and that is just not realistic. Because the thank you process is so time consuming (word-processing, photocopying, postage, etc) it’s MUCH more efficient to batch them in a regularly scheduled time slot. The original, paper source documents get dumped into a To File box to be handled at a later date when batch filing is done.
For the unusual, special situation gift: again, I think you’re right; for whatever reason (the circumstance, your experience, your instinct) you know that you’ve got to handle it differently, so in fact, it IS a project. Re-read your description and you’ll see that you’ve described a project exactly. So I think you need to deal with it as a project, and overcome your resistance to that idea. Once you’ve identified what characteristics make it a multi-step project, you have your GTD system in place to handle it.
As for monthly reports: I use a two-step process here as well. 1) I have a recurring task to run reports for the previous month on the first workday of the following month. The reports I want are set up as a template. 2) Reviewing and reconciling the reports is a separate recurring task that I schedule time for during the first week of each month (usually within a few days of when I run the report). I do this religiously. It is so much easier to reconcile and correct one month’s worth of gifts at a time. SO much easier and less painful than trying to re-construct things and correct them when the auditor arrives next year….