I only did a very quick test, and adding ¿¡ to symbols_word_connectors for es appears to help.
But lacking Spanish language skills I can't test properly and tell whether it may have unintended side effects, as isWordConnector is checked in many places.

Tell me what to do, what tests you need, and I can do it.

All I did was adding a file donottranslate-config-spacing-and-punctuations.xml to app/src/main/res/values-es with content

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">
    <string name="symbols_word_connectors">\'-¡¿</string>
</resources>

Can you compile the app yourself? If not I can upload an APK.

I never compiled an APK before 😅

Hm, I did some more test and noticed that if you tap the word after it's written, OpenBoard with my modification will show ¿Donde as center suggestion, while it should obviously be Donde...
So only while you write it looks good, but my initial approach definitely was not a solution.

Anyway, when adding ¿¡ to symbols_word_separators instead of symbols_word_connectors it appears to work.

APK (zipped because GitHub doesn't allow attaching APK): openboard_es_fix.zip
Note that you will have to uninstall OpenBoard, because the signing key does not match. If possible you should back up the original app before you try this version, so you don't lose your settings and typed word history (personal dictionary is safe).

Working like a charm so far