Do you think I can use XSLT to do this more or less easily?

Yes, XSLT 3 can do it easily, using tokenize to split the segments and
then grouping to group the different segments together:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
	xmlns:xs="http://www.w3.org/2001/XMLSchema"
	exclude-result-prefixes="#all"
	version="3.0">

  <xsl:template match="tu">
      <xsl:variable name="split">
          <xsl:apply-templates mode="split"/>
      </xsl:variable>
      <xsl:for-each-group select="$split/tuv/seg" group-by="position()
mod count($split/tuv[1]/seg)">
          <tu tuid="{position()}">
              <xsl:apply-templates select="current-group()/snapshot()/.."/>
          </tu>
      </xsl:for-each-group>
  </xsl:template>

  <xsl:template match="seg" expand-text="yes" mode="split">
      <xsl:for-each select="tokenize(., '(' || $lb || ')+')">
          <seg>{.}</seg>
      </xsl:for-each>
  </xsl:template>