After a dozen years off DnD, I came back to DM 5e.
I'm quite new to 5e, so I'm asking this forum to help.
Usually extensive past experience running AD&D or the like back in the day is a good preparation for jumping in and running 5e. It might've helped to have played for a bit, first, but if you were running good games in the TSR era, you should be able to do so again in 5e.
DMing now, as then, is more art than science, and, IMHO, it's like riding a bicycle.
However, they have created unoptimized characters.....
We were using 4d6dl in a row, best of three, as a method for stat generation.
Roll-in-order is likely to give you un-optimized characters, yes. It can be good for a group that's too prone to optimization, for instance.
Pregens might've been a better way to go with this group.
There were a couple of early deaths in the party.
Not unusual for 1st level in 5e.
Encounters that seem perfectly reasonable early on, like a bunch of kobolds, can turn quite deadly because of 5e 'Bounded Accuracy' contrasted with AD&D - not just rapid scaling of the attack-matrix/THAC0 on the player side, but high initial AC relative to less-than-1HD humanoids' ability to hit. The dynamic for a 1st level party back in the day was that enemies like kobolds or skeletons or whatever would have a hard time hitting the splint/banded-and-shield front line, while turn undead or the sleep spell would slash their numbers efficiently. So the party could handle being outnumbered by the right kind of opponents. Even facing a same-size group of theoretically inferior enemies at 1st level, now, can go south fast. If they're outnumbered, so much the worse.
The opposition that party is facing is highly varied. Indoors/outdoors combats. I can go into more detail if needed.
Highly varied encounters are a good way to break an optimized party out of a tactical rut, but can be too challenging for an un-optimized party with inexperienced/casual players.
The party can't figure out what spells to prepare, how to engage enemies and how to level up.
D&D does have a learning curve and the flexibility and versatility that makes casters so powerful when used optimally can also end up a trap when the player isn't up to the challenges. That's why the tradition is to start with a simple, forgiving (tough), class, like a Fighter or Barbarian, and move on when you've gotten the system down to a degree, and get bored with it.
Unfortunately, most of my observations don't help you. ;(
I will offer characters to switch skills/spells/feats/abilities at level 4.
Since your players aren't optimizers, and have built the characters they want, that may not help a lot. But if they are wishing they'd chosen differently that'd be a fine idea.
So while generally true - Bless is usually better than swinging a mace, but, against softer opponents, it might not be worth the slot/concentration.
Bless is a critical part of some party-buffing combos, typically involving leveraging a party heavy with Extra Attack and using specific feats. Doesn't apply to this party so much. They should be looking at optimal use of offensive (not simply damaging) spells.
Though, it really depends on the types of encounters they're facing, and other than 'varied' I don't think we've heard too much about that, yet? One thing that might help with them choosing spells is telegraphing challenges a bit. Put them in more situations where they know what they'll be facing going in and the Cleric & Druid can pick their spells to fit, perhaps?