Sharing this for anyone in the future who runs into the same issue I was having. This was my solution:
//TablingTimeListingsBox = the list box where people select their times
//FridayTabling = the data table that you are updating
//TablingDateSelector = the dropdown menu of dates
//This process relies on the first five digits of each listbox listing, and checks to see if they equal a corresponding text in a switch statement. If they do, then it updates that specific record. It isn't as dynamic as I would've prefered, but it works! I should note that the space for all dates 1:00 and onward is important, as "1:00" /= "1:00 " and my listings include that. Anyone else who runs into this, be sure you are particular on such details!
Notify("This may take a few seconds, please don't close the app.",NotificationType.Information,10000);
ForAll(TablingTimeListingsBox.SelectedItems,Switch(Left(ThisRecord.Value,5),
"12:30",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '12:30':ThisRecord.'12:30'&", "&SignUpAsInput.Text}),
"12:45",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '12:45':ThisRecord.'12:45'&", "&SignUpAsInput.Text}),
"1:00 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '13:00':ThisRecord.'13:00'&", "&SignUpAsInput.Text}),
"1:15 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '13:15':ThisRecord.'13:15'&", "&SignUpAsInput.Text}),
"1:30 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '13:30':ThisRecord.'13:30'&", "&SignUpAsInput.Text}),
"1:45 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '13:45':ThisRecord.'13:45'&", "&SignUpAsInput.Text}),
"2:00 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '14:00':ThisRecord.'14:00'&", "&SignUpAsInput.Text}),
"2:15 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '14:15':ThisRecord.'14:15'&", "&SignUpAsInput.Text}),
"2:30 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '14:30':ThisRecord.'14:30'&", "&SignUpAsInput.Text}),
"2:45 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '14:45':ThisRecord.'14:45'&", "&SignUpAsInput.Text}),
"3:00 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '15:00':ThisRecord.'15:00'&", "&SignUpAsInput.Text}),
"3:15 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '15:15':ThisRecord.'15:15'&", "&SignUpAsInput.Text}),
"3:30 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '15:30':ThisRecord.'15:30'&", "&SignUpAsInput.Text}),
"3:45 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '15:45':ThisRecord.'15:45'&", "&SignUpAsInput.Text}),
"4:00 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '16:00':ThisRecord.'16:00'&", "&SignUpAsInput.Text}),
"4:15 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '16:15':ThisRecord.'16:15'&", "&SignUpAsInput.Text}),
"4:30 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '16:30':ThisRecord.'16:30'&", "&SignUpAsInput.Text}),
"4:45 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '16:45':ThisRecord.'16:45'&", "&SignUpAsInput.Text}),
"5:00 ",UpdateIf(FridayTabling,ThisRecord.ID = TablingDateSelector.Selected.ID,{ '17:00':ThisRecord.'17:00'&", "&SignUpAsInput.Text})
));