Rail Europe or Strictly Train Operators

I don't know if they are still in business, but there used to be a company called "Euraide" which would help you get reservations for European trains.

Lyon Part Dieu => Geneve is a TER with no reservations.
Geneve => Zuerich HB is a Swiss IC train that does not require reservations, but you can buy them for 5 CHF if you like from SBB.

The fact that SNCF owns Rail Europe may have something to do with it.

Ohhh, I didn't know Rail Europe was actually owned by SNCF, that makes a bit more sense now. I'm only making reservations that involve my big travel days; going between countries, etc. at the moment especially since I haven't been to the EU in June and don't know how the crowds are going to be.

There are 2 direct trains from Geneva to Zurich every hour. The trains actually originate at Geneva airport, so you will be one of the first people on board. Because trains are so frequent, Swiss seldom buy seat reservations, thus it is pretty safe to just get on board and find a seat. If someone did reserve a seat, it will show on the electronic indicator above the seat.

Toulouse-Lyon you could also get at the station. In Switzerland I wouldn’t bother with reservations.

Regarding buying reservations from other operators: anyone with a link in the European reservation system can reserve seats for you. It’s just that some have easier interfaces than others. SNCF for example doesn’t really want people with Eurail passes on their trains.

In France you don't want normal seat reservations, all IC, EC, some TER, all TGV, require special rationed Pass Holder reservations which are rarer (even if there are still seats on the train) and rationed, especially rationed on international trains.

Don't buy the wrong thing - you've already spent enough for the pass, you don't want to waste more money and then still have to buy the correct pass holder reservations.

France is much more difficult for pass holders than other countries.

If, for whatever reason, you are changing trains and stations at Lille- you may find it easier just to buy an ordinary ticket for that leg to Paris. It is also interesting that, buying a normal ticket, 1st class is just 2 Euro more than 2nd class on many trains. Booking on SNCF that includes your seat reservation- which you get to choose off a plan- either upstairs (Haute) or downstairs (basse).

If you are taking the 0704 train from London that 33 Euro 2nd/35 Euro 1st class fare applies to the train you are catching- so that may be your easiest option to upgrade to 1st class for that leg on your own separate ticket.

buying tickets may be excess spending as the OP has a railpass

Should I be making seat reservations for IC and EC trains even though they say they're optional? Some of the other travel I will be doing is between Zurich and Munich, Munich and Vienna, Zurich and Frankfurt

Up to you, but they could be useful on certain trains. Zurich to Munich automatically comes with a seat reservation when you buy a ticket. Munich-Vienna and Zurich-Frankfurt can also get full depending on the timing. For these, I would check the DB app which will show the projected loading a week before. If it is showing heavy (3 stick figures), buy seat reservations as they are only 4.90 EUR.

With the frequency of trains in the DACH area it is generally not necessary to reserve far in advance. You will be perfectly fine reserving a ticket at the station the day before. Trains are mass transit here. There are always tickets. It is rare for all seats to get reserved. You book in advance for the discounts, not because "tickets might run out". If there is one place where you should let go of your seat anxiety it is here...

DACH area

???

DACH is an acronym for D (Deutschland- Germany), A (Austria), CH (Switzerland- Confoederatio Helvetica/The Helvetic Republic )

https://www.statista.com/topics/4623/dach-countries/#topicOverview