Eurail Global Pass in September

Are you sure you are stuck with the pass? Can't you get a 90% or 85% refund? That might be worth it, you'd have to do the math.

You only need reservations in Italy, and then only for Long distance trains (High Speed and EC), NOT for local trains (Regionale).
No reservations in Switzerland.

Luzern to Menaggio - No rail line to Menaggio. You need to go to Lugano, and then get a local bus to Menaggio, which is not covered by your pass.
Info on how to get to Menaggio here: http://www.menaggio.com/contenuti.php?PADRE=13
Bus timetable here: http://www.menaggio.com/newmedia/pdf_turistico/come%20raggiungerci/C12est2019.pdf

Ortisei also requires a bus to reach it.
Getting there from Menaggio is convoluted and takes a long time. See here: https://www.rome2rio.com/map/Menaggio/Urtij%C3%ABi

Thank You, But should I focus on making the reservations now? Are trains usually full in late September? Or could I maybe get away with making the reservations at the station or on the app a few hours or days ahead?

You could make one for Venice to Milan if you have a specific time and date

Drobcak, the trains never fill up (nearly never, times like Easter may be the exception), the prices just go up as you get nearer the date, you can always (nearly) buy a ticket on the day at the station, it just costs more.

The above statement applies to buying tickets, you aren't doing that, you have a pass and just want to buy a reservation separately. That is not a normal option (normally people buy tickets), I have never done that so I cannot comment on what restrictions there are. Obviously the rail companies prefer people buying tickets above those travelling on passes for which they only get a small cut, they may restrict pass-holders.

You will definitely need a reservation for Venice to Milan, and possibly for the rail portions of Menaggio - Ortisei and Ortisei - Venice, depending on which trains you take.
I would get your reservations from whoever sold you the pass.

When you travel with a Eurail pass in Italy needing or not needing reservations has little to do with how full or empty the train is but almost exclusively what grade of train you travel on.

Fast intercity trains are either Italo (the competition to the national railway company who have generally newer and often cheaper trains but who don't cover the whole country), Freccia trains (in 3 different colours indicated in their names, part of Trenitalia the national railway), or InterCities (the slowest of the Trenitalia faster trains, usually using quite old rolling stock).

You can't use Italo trains with the pass - they don't accept passes.

Passholders are required to purchase a reservation (included in the price of normal tickets but extra for passholders) for every leg of FReccia or IC trains. With the reservations you sit in car and seat on the reservation. All seats are reserved.

There are two other grades of trains in most of Italy. The Regionale and RV Regionale Veloce. These are like buses, there are no reservations needed or offered. You get on and find a seat somewhere, and if there are no seats you stand until one becomes available. Passholders don't need to buy reservations on R or RV trains, your pass is sufficient. These are the only trains in Italy where passholders retain freedom to hop on and hop off.

I travelled from Milan to Venice and from Lucerne to Milan on a global pass in May. You don’t need seat reservations within Switzerland, but you will need them on the EC trains entering or departing Italy.

Within Italy you will need seat reservations for the FR, IC, or EC trains.

You also can no longer make seat reservations via the ticket machines or the Trenitalia website/app. You have to either go into the Trenitalia service centre at the station (or SBB in Switzerland), take a number and wait to be served or you can try book a seat via italiarail.com or the Eurail app and pay an extra service fee. I had to wait around 20-30mins for my number to be called so keep this in mind and give yourself plenty of time in case there is a large queue.

Since you would need to go into the service centre to get your pass validated, I would suggest making your reservations at the same time for any trains that you know you want to lock in.

Thanks for the reminder about the EuroCity EC trains. They are basically IC trains which cross borders.

And, yes, I agree - no reservations required for the Swiss portion of the train but a reservation is required for the Italian part of the trip on EC trains.