i think going forward, you would want to either use an observation field that is unique to your effort (ex. OMDL_voucher=OMDL00001) or use an additional observation field which you set to a particular value that is unique to your effort (ex. voucher_set=OMDL). alternatively, or in addition to that, it may be useful to include these in a specific traditional project, as thomaseverest suggested above.
once you have a unique observation field, field value, or project, then you will be able to use the unique field, field value, or project to filter for your entire set of records, rather than filtering for individual observation IDs.
so then what to do about all the observation records that have already been processed? you could use the API to update those records with a new field, field value, and/or project.
here’s quick description of generally how to execute multiple API POST requests via curl in Windows command line / batch: https://forum.inaturalist.org/t/unsubscribe-from-multiple-items-at-one-time-on-subscriptions-page/33728/6. you could follow this general process if you use Windows, too, or you adapt it for bash or R or whatever other scripting tool you want to use.
here and in subsequent posts in the same thread, i’ve described specifically how to add/remove observations from a project using Windows command / batch: https://forum.inaturalist.org/t/bulk-selection-of-observations-to-add-to-a-project/1747/70.
i haven’t previously made an example for adding observation fields or field values, but the curl command for this sort of action should look something like this: curl -X POST "https://api.inaturalist.org/v1/observation_field_values" -H "Content-Type: application/json" -H "Accept: application/json" -H "Authorization: JWT" -d "{\"observation_field_value\": {\"observation_id\": 0, \"observation_field_id\": 0, \"value\": "\string\"}"