BB has cowardly tendencies but he’s also exceptionally lucky and probably doesn’t give a fuck about raw strength as long as his goals are achieved.
Reminder that Blackbeard's raw strength and endurance is fucking insane.
-He gave Shanks his scar with only a claw weapon
--Blackbeard had no DF powers
--This was a straight fight with no sneak attacks. The Vivre Card databook says Shanks was only caught off-guard by how strong Teech was. Ace's Story says the fight happened in the East Blue, and the Whitebeard Pirates had no idea it was one of them.
-He fucked up Ace with nothing
but raw strength and his fists.
--He only had the power to absorb powers and other shit, and also had his damage handicap as you mentioned, the presence of which did not prevent him from being the only one standing and winded while Ace was exhausted towards the end of the fight.
-He tanked several quakes from Whitebeard. He may have been shitting himself (in the anime, at least) but he was still able to move quick enough to initiate a counterattack and then recover to fight Sengoku later.
--The only other person we've seen do this is Akainu, who not only
does not have the damage thing BB does, but is flat out said by Oda to be so strong, he could find the One Piece within a year.
-He's also not just lucky, he's fucking
smart
--One of his SBS-stated hobbies:
archaeology
--His planning has enough leeway (and your aformentioned luck) that he could pivot whenever trouble affected it ----the Dark-Dark Fruit search, killing Luffy for the Warlord position and apprehending Ace instead, sending Jesus Burgess to get the Flame-Flame Fruit and ending up with Baltigo's location, etc)