If you know when you'll be travelling and don't need to be flexible, then you can make a comparison between pass + reservations and normal tickets.

You can easily see for yourself via the websites of example DB BAHN and SBB, whether you can make the journeys you want to make without a high-speed train.

You can make all connections without reservation costs, except to and from London. Provided you have enough time!

The main benefit is the flexibility! You can decide 1-2 days before (or even just on the spot if it is not high season etc.) where you wanna go. Get a reservation (if neccessary) and hop on. For me the Interrail pass pays off almost always. But if you have such a fixed mainstream schedule, then you can also try it with normal tickets, might be cheaper if you know the exact dates of traveling, but will limit your flexibility - if you like one place a lot and wanna stay longer, you will (depending on what kind of ticket you booked) not be able to change it without a lot of extra costs…. same goes for if you don’t like a place and wanna leave it as soon as possible.

at least AMS-Berlin is with voluntary REServation-with the risk of having to stand a short distance if all is full by reserved seats.

Other sectors CAN be made, but much slower and with more changes, by unreserved local trains.

You have simply chosen a routing where this problem is quite big-Spain (which you avoid), then France with its TGV then EuroStar are the main culprits. Be flexible, discover EUrope and choose other routings! As rian explains-if you want to stick to that, maybe advance fixed bookings are a better choice.

You can also save a bundle by using long-distance BUses instead, f.e. the ever present Flix. Often their fare is lower as only the extra REservation on EU/InterRail

This is also confusing to me because when you reserve seats for leg 2 of the trip, but leg 1 doesn’t need reservations...there seems  no way to be sure you’ll be able to get on to leg 1 with your pass...it might be full...and then you miss your leg 2 paid reservation? :( 

It all depends on the route and the trains.
If you ask in this community with few details the experienced travelers may have a good advice for you.