Ovinomancer
No flips for you!
Math time. Not including blesses saving throw bonus, just damage.
Worst condition. 1 attacker with 1d8+3.
1d8+3 * .125 = 0.9375 damage per round.
Moderate condition. 2 attackers with 1d8+4 and 2 attacks, 1 with sharp shooter.
(2d8+18) * .125 * 2= 6.75 damage per round.
Optimal bless condition of 3 attackers with 1d8+5, sharpshooter, and 4 attacks.
(3d8+45) * .125 * 4 = 29.25 damage per round.
Spiritual Weapon.
1d8+3 * .60 = 4.5 damage per round. No concentration, cast / maintain with a bonus action. (so 1 extra sacred flame).
Spiritual Guardian.
3d8 * .6 + 3d8*.4/2 = 10.8 damage per round. AoE.
So bless is only broken if you build around it. Though i still think it should be moved up to a level 2 spell.
Quibbles with your math.
1) Spiritual Guardian should be doing 9.2 damage by your calculation, not 10.8.
2) your moderate condition is pretty lackluster. Except in very small parties, the odds of only blessing 2 people is pretty small, and would be the exception rather than the rule. Also, at 5th, my party had a ranger (slayer), polearm paladin, and a monk as the front line (supported by a cleric and a bard). The average damage for the ranger was 30.5/rnd (with mark and a +1 bow, so a 2d8+d6+5 and a d8+d6+5). The paladin was 25.5 (d10+4+d10+4+d4+4), and the monk was 20.5 (d6+3 x3) [see, one of them wasn't optimized!, actually only the ranger had a true 18 stat, the paladin had gauntlets, and the monk a +3 dex.]
So a grand total of a possible 76.5 damage a round, without spells (other than mark), crits, smites, or flurries. By that math, bless increases average damage per round by a little more than 9.5. That tops everything except a Spiritual Guardians that has more than one person in the AOE. Over the course of a 3 round combat, that's almost 30 more damage dealt.
Bless is, by far, not the best spell. It's conditional, highly contingent on party makeup, and has strong competition for the concentration slot at higher levels. But your analysis seems heavily slanted towards downplaying the merit that it does have. It's a solid spell, a workhorse even, and not some trivially useful spell that appears better than it is. It's exactly as good as it appears.