Making a balanced boss

I have a plan to have my party fight against a former party member in a flash forward boss encounter. They’re based on a battle smith artificer but the feature is that one half of the duo becomes stronger when the other is felled in combat. It’s a level 6 party of 4 players (evokes wizard, open hand monk arid land Druid and fiend warlock) and they’re reasonably capable when it comes to fighting so I’d thought I’d amp up the challenge a tad but I’m not fully sure whether giving the boss legendary/lair actions would break things too much. Is level 6 ok enough to start with legendary actions?