Just use a static variable id like:
<dmx-value dmx-bind:value="0" id="var_1"></dmx-value>
and reference it using its static id in the code:
<p class="card-text mb-xxl-3" dmx-show="var_1.value==0">{{fb_review.trunc(300, true, '...')}}
<button id="btn3" class="btn btn-outline-info" dmx-on:click="var_1.setValue(1)">read more</button>
</p>
<p class="card-text mb-xxl-3 mb-md-3 mb-3" dmx-hide="var_1.value==0">{{fb_review}} <button id="btn5" class="btn btn-outline-info" dmx-on:click="var_1.setValue(0)">read less</button></p>