I'm updating a legacy(12 y/o) system and one of the requirements is a cta that opens a model which lets the user make multiple selections, aswell as, typing an input that then gets added to a summary of added items. I was just wondering if there was a cleaner design pattern for this use case.
Checkboxes are often easier to use than a multi-select, and also easier to make accessible and mobile friendly (especially if it will be web-based or an app). Perhaps your concern, though, is that your list of items is very long, in which case you could think about grouping the items (under subheadings, in separate boxes, etc. based on your context) or providing some kind of filtering or search within the list.
If your modal contains enough content, consider converting the modal into a full web page of its own. Its easier to scroll, and more accessible.