Hi @dsab81 ,
Regarding the needs that you mentioned, I think PowerApps canvas app could achieve your needs.
On your side, you should replace the original "To" Text Input Box with a ComboBox control. Then bind the ComboBox control to your SP List which consists of "managerName" and "managerEmail". Set the Items property of the ComboBox to your SP List, then specify the managerName column as 'Primary text' and 'SearchField' in this ComboBox.
I have made a test on my side, please try the following workaround:
My SP List as below:

On your side, please your SP List as data source into your app. Then hide the original TextSearchBox1, AddIcon1 and SearchIcon1 controls:

Then add a ComboBox control under the "To" Label, name it as "PeopleComboBox". Set the Items property of this ComboBox to following:
'Manager List' // you should replace it with your own SP List
then specify managerName as 'Primary text' and 'SearchField':

enable "Allow Multiple selections" option for this ComboBox. Then set the OnSelect property of the "Send Email" Icon to following:
Microsoft365Outlook.SendEmail(
Concat(PeopleComboBox.SelectedItems,managerEmail & ";"), // Modify formula here
TextEmailSubject1.Text,
TextEmailMessage1.Text,
{Importance:"Normal"}
);
Reset(TextEmailSubject1);
Reset(TextEmailMessage1);
Reset(PeopleComboBox) // Reset PeopleComboBox

After that, when you click the PeopleComboBox, it would show up the Person list from your SP List, you could select one or Multiple options there.
Please try above solution, hope it could solve your problem.
Regards,