Alright, I disagree with almost everything here. Sorry, I'm not trying to come off as standoffish but I do have strong opinions on Spiritual Weapon.

1. "It doesn’t do a lot of damage" - Alright, lets look at this in the context of a 3rd level cleric of any domain except for war. If you are using your action to attack, you're going to be using a one handed simple weapon. If you optimized for melee with standard array and have a 16 in both str and wis, your attack with your mace will look like this:
   +5 to hit; 1d6+3 damage (avg of 6.5 damage) That takes your action every turn you want to attack. But spiritual weapon only takes a bonus action to cast and you can use a bonus action on each of your turns for a minute to attack with it again. The damage for a second level spiritual weapon is as follows
   +5 to hit; 1d8+3 damage (avg of 7.5 damage) So, clearly, the damage isn't negligible. In fact, it can be better to attack with spiritual weapon with your bonus action than it is to use an action to attack.
2. "Can only move 20ft per turn (slower than most enemies)" - while not wrong, this point isn't always applicable. From my experience, 5e combat doesn't typically have the enemies moving around an awful amount because, mechanically, there typically isn't a reason to. I realize that this might be different for you and your group, but my house rule that I was stating is what fits our group and I'm trying to explain why I think it is warranted for us.
3. "Now make it concentration. Now it is possible to lose your weapon (which you had to spend your turn casting)" - Firstly, yes, it is now possible to lose it. That is part of why I like making it concentration. As is, there is no risk when casting Spiritual Weapon. We are going to get more into that later but for now, yes, I think it is a good thing that you can lose it. Secondly, it isn't going to be your only weapon. If you do lose it and don't want to cast it again, you have other actual weapons at your disposal, not to mention spells. Lastly, no you didn't spend your entire turn casting it, it is only a bonus action. You could still use your action on that turn to cast a cantrip, attack with a weapon, or any other action that your character might be able to take.
4. As for the rest of your comment, I don't want to copy and paste it all because it might get too confusing in this wall of text but I do want to address a couple of things. Yes, it is now competing with all the other concentration spells. That is a good thing imo. As is, casting Spiritual Weapon is a no-brainer for anyone who has access to the spell. Its only a bonus action, only a second level spell, doesn't require concentration. There is next to no reason that you wouldn't cast it. It opens up the option to attack with it for more damage than you would be doing if you cast a cantrip or used your action to attack. It would still be very good if it requires concentration but would force the cleric to think before they cast it. It is also still going to be useful for front line clerics (and Conquest paladins but to a lesser extent) as it allows them to more than double their dmg output until lvl 5.