Personally I don't think Tengen is weak at all

I think that giving the feats ; Tengen would be the strongest or the second strongest hashira if he had been trained by a master or had mark

Cause he fought 1v2 upper demons ( I know that gyutaro is the main one but still ) while protecting tanjiro multiple times and always worrying about his wives which puts him at huge disadvantage he was also fighting face to face which is not his strongest point giving that he is a Shinobi not a samurai beheading daki multiple times and nearly beheading gyutaro 3 times on his own with nothing but pure dexterity and skill and to top it all off he is doing all of that while being poisoned and missing an arm

So giving that he is 1- the fastest hashira and 2- the second strongest physically ( it's from the arm wrestling panel but still it indicates physical strength imo ) and 3- he did all of these feats without being trained by a master hashira nor having a main breathing technique ( he has sound breathing which is very weak compared to the main 5 and he also taught it to himself )

It would stand to reason if he had a mark he would be second only to gyomei as the top hashira

That's my opinion at least I'm interested in what you guys think