We should have an optional feature to be able to open new directory on the the same/single instance of dolphin over tabs this patch implement it and make it available through settings page.
Note 1: this code is tested with latest v17 (v17.12.3) my distro stable release use KF5 v5.42 and have not yet switched to newer release (its for soon) this makes testing/implementing this on dolphin v18 big time consuming for me... any way it should be easy to adapt the code for v18.
Note 2: the following additional diff may be useful
diff --git a/src/org.kde.dolphin.desktop b/src/org.kde.dolphin.desktop
index f187cef..f6418d2 100755
@@ -42,7 +42,7 @@ Name[uk]=Dolphin
+Exec=dbus-launch dolphin "%u"
Note 3: i just noticed that i forget the root patching on this diff... (not required anyway as dolphin with root have been fixed in v18... i was just commenting out root code in main.cpp)