I would create a custom action that instead takes a date/time variable as input.
Then in the script use add days method, and evaluate if the new date is within the same month or not, that way you would know if it's the last thursday, or whatever day, in any given month or not.
This flow could run weekly.
Here's a how I'm thinking;
var inputDate = someDateToCheck;
var checkLastDayOfMonth = inputDate.addDays(7);
if (checkLastDayOfMonth.getMonth() > inputDate.getMonth() )
return true;
else
return false;
The second example when wanting to have the first weekday of the month, is a bit more complex. I can think of two different approaches.
Either you run it daily, and in the script evaluate if it is the first weekday of the month (can also be done with GlideDateTime functions), but it would also require you to evaluate and know if the job already has been run a particular month or not. I'm not sure if the second part is doable, I don't know if you can check this somehow.
The other option is to run the Flow monthly, on the first day of the month, and in a custom script evaluate if its a weekday or not. If it's a weekend, the action should return a number of hours the flow should wait (in an wait action) until continuing the trigger.
