travelling to Thailand as American/Belgian Dual citizen

US nationals entering thailand with a valid US passport, and a valid return ticket, do not need visas for entering Thailand.

They also do not need a visa for Belgium as the EU has a visa waiver for US citizens.

So you should be fine to leave and return, provided your US passport is valid and provided you have a return ticket.

You will need a tourist visa for stays of > 30 days.

U.S. citizens carrying a tourist passport and in possession of an onward or return airline ticket do not require a visa to enter Thailand. The passport must have at least six months validity remaining to be allowed entry. Upon entry, Thai immigration officials will place an immigration stamp in the passport permitting a 30-day stay in Thailand if arriving by air or land. This time limit may subsequently be extended for an additional 30 days by paying a 1,900 baht fee to the Thai Immigration Bureau office. The headquarters of the Thai Immigration Bureau is located at Government Center Chaengwattana Building B, No. 120, Moo 3, Chaengwattana Road, Tungsonghong Sub-District, Laksi District, Bangkok 10210, 66-2-287-3101 through 66-2-287-3110. For more information or additional Thai Immigration Bureau locations, please consult the Thai Immigration Bureau website.

https://th.usembassy.gov/u-s-citizen-services/thai-visas-americans/

Edited to add: You will need to show your Belgian ID when you exit NL, otherwise you will be questioned about your lack of a Schengen entry stamp. Your ID should be fine for this, as @CrazyDre addresses in their comment. And apparently you don't need to show your US passport on the European side, provided you have your Belgian ID.

Show the following documents:

1. At bag drop in Brussels (if applicable): US passport

2. At the gate in Brussels (if asked for ID): either document

3. At border control in Amsterdam: Belgian ID

4. At the gate in Amsterdam: US Passport

5. At border control in Bangkok: US passport

On the way back:

1. At bag drop in Bangkok (if applicable): either document
2. At the gate in Bangkok: either document
3. At border control on arrival in Europe: Belgian ID

So yes, this trip is 100% fine