Best Way to use my eurail select pass

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I bought (yes, stupidly) a eurail select pass which i plan to use across germany and benelux. Any ideas on how i can best use it to get the most value and avoid paying more? Thanking any and all in advance.

Well, answer a lot depends on your itinerary and how many travel days you actually have with your Eurail Select Pass. Generally it's a good idea simply to list all the potential journeys you intend to take and if you can't cover all of them with you pass travel days, then you should check "cheapest"/shortest journeys on the list for which it's will be ok to buy regular full fare standard ticket instead of using travel day from your pass. Majority of standard fares you can find on official page of German Railways (DB) - bahn.de
Regarding issues with seat reservations you more or less have nothing to worry here, since Germany and Benelux countries almost don't have trains with obligatory seat reservations (should be purchased on top of the pass), as result you can just hop on/hop off on almost any train without need to think about reserving a seat. Seat reservations obligatory only for ICE Sprinter trains in Germany and for Thalys/Fyra high-speed trains in Belgium/Netherlands

you cannot use Thalys with that pass.
FYRA is limited to some hi-speed trains In NL now only and these require-also with that pass-the 2,30 supplmt to be paid. it is also NON RES. This does NOT mean you can sit anywhre-other people may have reserved and these seats are marked-for teh sector for whcih. So use eyes and look before sitting down. in Germany FRI and SUN afternoon-eves can be very busy.

you cannot use Thalys with that pass.

Arrivans, are you sure? It seems that both Eurail & Interrail are still valid for Thalys, at least Thalys website confirms it