Not sure I understand how limiting smites to smites per round affects Damage imbalance. At level 11, a Paladin can cast a maximum of 10 smites between long rests. With 2 attacks per round and a "rough" 50% hit chance, A Paladin must make 10 Attack Actions between long rests to maximise damage. The DMG suggest (roughly) 2-3 encounters per SHORT rest, and that roughly each encounter lasts 5 turns on average. Therefore, you can expect the Paladin to tap out Smites (Using smite on Every hit) after 2 of the 3 encounters before a short rest, and then have nothing for the day.
By my count, it doesn't matter if it's twice per round for the first 5 attacks, or once over the first 10 rounds, Or 1 smite per encounter before a long rest. Over the course of a day (assuming 4.5 average on every Smite d8), you get an extra (29d8s) 130 points of damage per Long rest. Limiting the burst-i-ness does utterly nothing to change this.
To put that in perspective - using 3 short rests per long rest (call it 10 encounters), a fighter will make 50 attack actions (roughly) per long rest. With a +1 STR over a paladin (from an ASI), he can expect to hit 5% more often (increase damage by 5%) and do 1 point of damage extra per hit. A GWF averages 8.33 per dice roll plus STR. Lets say we're comparing 18 STR to 20 STR, soooo......
Paladin: 50 Attack actions @ 50% to hit = 50 Hits x 12.33 Damage + 130 Smite damage = 747 Damage per long rest
Fighter: 50 Attack actions @ 55% to hit (from +1 STR) = 55 Hits x 13.33 Damage = 733 Damage Damage per long rest
So a level 11 paladin's smite damage does the equivalent to +1 STR per long rest. And completely regardless of whether that's front loaded or not. Yeah a Paladin can Nova, but a fighter with +1 more STR does just as much damage. In fact, limiting Smite to Per Round ONLY matters if you make less attack actions per long rest that you have Smites - which just isn't going to happen in any sensible game.
