Why is level 20 your benchmark for having at least a half-decent build? How would you define half-decent?
I can confirm that the devs do want players still feeling like they have points they need to spend way beyond level 20. If the pace is too slow then there are options you can select to speed it up.
Generally, hitting level 20 by around day 14ish seems to be about the general pace-mark if you're aggressive with zombies and spawned hordes. That means I've already sunk 14 hours into a single game, and by then you should be far and away well into the starting strides of anything resembling a 'build'. Day 14 doesn't pull punches and the blows don't come any softer after that point and it's far too late to be half-assed at anything.
From a solo perspective, surviving the first week is a quick nut-punch to be sure you have the basics down, but not dying in the second wave comes down to supply and having spent on genuine must-have skills: Advanced engineering 1 & 2 (forge & workbench), Healing Factor 1 (core self-healing), Pack Mule 1, Master Chef 1 (because charred meat can be fatal), Miner 69'er 1 (Q1 tools are ass!). That's a MINIMUM amount of early-game functionality, not counting the need for a chem station, sustainable farming, advanced defenses, and medical supplies that are needed later down the road.
A solo Melee is MAD (multiple attribute dependency), particularly spears. You're talking heavy investment in Fortitude (HFactor, Tolerance), with a weapon choice very likely diverging into Strength (blunts & stamina management) and Agility (light armor, slashing). You certainly end up strapped for points for 'fun' takes like Lucky Looter, Hunting perks (Tracker/Hunter), Daring Adventurer.
My last run with heavy armor and a spear in a 2-player coop ended horribly. Heavy armor really doesn't progress (everything is ~10 armor val), doesn't significantly differ from light armor, and is crippling when you need to lug it in your inventory to a POI mission or travel on foot while wearing it. I gave up on the spear as anything but a backup melee weapon (where literally anything could have served that purpose) in favor of any gun I had, as the spears' poor damage, swing speed, and tendency to simply 'go missing' on accident proved it to be a gross liability, and that's saying something when the most historically advantageous weapon ranks worse than a jar of beer and two fists and the fireaxe which magically lost its weapon status in A18.
By level 20, I couldn't tank for ♥♥♥♥, I couldn't melee for ♥♥♥♥, couldn't so much as build a forge, I was constantly dying of starvation and thirst, and was 'that guy' that was 200m behind everyone else because of the armor penalties and hunger-inspired stamina drain.
