For using clothing buffs, I just tested & found that even if the buff didn't apply after reload they can remain assigned to the plot. With
Cheat Terminal I can add legendary effect to any item so I added Cunning(+1 AGI & +1 PER) including Military Fatigues, Pack Feather Necklace, etc and boosted a settler from 1 AGI to 11. Assigned them to Advanced Defenses then removed the new equipment putting them back to 1 AGI. Fast travelled away and waited 24h, fast travelled back and they remained assigned to it. Somewhat exploitable but overall probably a positive thing since it means we can work around the base game bug where clothing buffs don't always continue to apply properly after cell reloads have happened.
Even without additional mods there are some pretty good options for starting out. Base game
Fallout 4 Companions tend to have high SPECIAL stats. Piper, MacCready, Nick Valentine, Deacon, Preston Garvey, Codsworth, Curie, Cait, Strong & Old Longfellow can join the player fairly easily early on and can be dismissed to a settlement. They can be useful as early game options for skilled/gifted requiring plots.
SS2 characters tend to have high SPECIAL stats as well, if you play through the quests you'll get a fair number of unique characters with good stats. Stodge for instance had 11 INT and reasonable values in other areas. Do the side quests The Ron gives you, It Came From Planet X & Gray Matters, and you can get a few more as well. Even besides the questline, there's quite a few additional unlockable settlers that can show up at any settlement you have with an active recruitment beacon once conditions are met. Claim at least 2 settlements and you should be eligible for a few, build up & raise happiness at one of them and you should become eligible for more. Training Recreation plots can also increase SPECIAL, the rate seems to get a fair bit better at L3 and some of them can have multiple occupants at L2/L3.
Companion mods can add another character with great stats, some examples include
Heather Casdin,
Ellen the Cartographer, Companion Ivy. For even more, the mod
Better Settlers may be worth a try.