it seems that that xml files does not only contains text but also other data, by the end of the file seems like a toc or data offsets. so simply editing text and changing the file size will break the game... you need to reverse engineer the file... maybe the devs of
kuriimu can help you make a plugin for that file.
but another problem might come out since usually, full Japanese SRW games have no font for half-width characters (ABCDEF) and only has full-width English characters (ABCDEF) it's been like that since the PS2/PS3 days. When you place a normal half-width characters (ABCDEF) whether its acsii/utf8 it will still interpret it as a full-width text and will come out as s unreadable Japanese text or no font at all.
here's an example of an edit without changing the size so it won't crash
Original text: (1ターンの間、最終命中率と最終回避率が30%アップします。)
Modified text using half-width text: (The final hit rate and the final evasion rate are up 30% d) and it came out as garbage
Modified text using full-width (shift-jis) text: (BGMBGMBGMBGMBGMBGMBGMBGMBGM)