...Overall I would say Blur is one of the top 10 spells cast period along with shield, Hex, magic missile and healing word. Among 2nd level spells the only spell I can think of that are cast more often is shatter and misty step, and misty step only because it comes on an awesome feat making it available to everyone.
This is ... very different ... than my experience at a wide number of game tables. It also does not match what we saw from the D&DBeyond metrics previously released. Spells I see cast at 2nd level FAR more often than Blur? Misty Step, Mirror Image, Invisibility, Suggestion, Mind Whip, Phantasmal Force, Scorching Ray, Pass without Trace, Spiritual Weapon, Aid, Enlarge, Darkness, Vortex Warp (as of recent), Lesser Restoration ... and those are just the ones cast FAR more often. Blur, being a defensive spell consuming concentration, really does not see much play - and if you seek out the guides that are out there such as Treatmonk's, you'll see there are a lot that share the opinion that this is a mediocre spell, at best, in 5E.
Yes all the time - Protection from Good and Evil, Blur, Spirit Guardians, Greater Invisibility
I think those are the most commonly cast concentration spells of 1st, 2nd, 3rd and 4th level. Can you name any concentration spells of equal level that are cast more often than these? All of them can target only the caster if you play with your changes.
Under the actual rules I use at my home table, the self with a radius for Spirit Guardians removes it from contention.
However, your opinions are out of whack with a lot of folks. Protection from Good and Evil and Blur are low rated spells by many of the guides, and by myself. For 2nd level, I see Phantasmal Force, Darkness, Enlarge, and Suggestion cast 10 times (each) for every cast of Blur. For 1st level spells, I don't really see a lot of conentration spells cast outside of Hex, Hunter's Mark and Silent Image. Sometimes Fog Cloud and Hideous Laughter. By mid levels, those first level slots are often relegated to Healing, Shield, Absorb Elements, Mage Armor, and Silvery Barbs 90% of the time.
I think you have to go all the way to 5th level and Animate Objects to find a concentration spell that does not target the caster and is cast more often.
I can name 40 spells from levels 1 to 4 that are typically cast one someone outside the caster and use concentration that are cast more often than blur and protection G/E.
Greater invisibility makes you immune to the vast majority of enemy spells that enemies could target you with. It is extremely powerful against any enemy caster that does not have blindsight. You are also taking away one of the main counters to it - throw an AOE and try to break concentration.
Yes - and I'm cutting the duration and requring a higher level slot.
This is aside from a new tactic of spamming regular 2nd level invisibility as a bonus action while you use your action for something else (or just stay invisible while you maintain concentration-free spirit guardians).
3rd level - remember that bump up in spell level. Also,
you're forgetting the opportunity cost. There is a reason why you do not see sorcerers spamming invisibility with quickened spell.
You're speculating. I'm speaking from experience with these for several years. And, a lot of the judgments you're using to support your opinion are contrary to the popular opinions expressed by the people that wrote the guides for wizards in 5E, and contradict what I see at the tables, and contradict what we have seen from data from D&D Beyond about what spells are actually being used.
Good luck to you, but we're going around in circles here and your suppositions just do not match with my experiences have actually had these at my table extensively, and my experience when I've been allowed to use them at other tables.