Ngl, I find takes that are "Zenitsu experienced no growth or development prior to the Hashira Training arc" and "Nezuko isn't a character she's a plot device"...baffling, and it's not a take that canon supports.
Zenitsu is introduced as a "coward"--though this is quickly delved into and shown to be a case of extremely debilitating anxiety, low self-esteem, and self-hatred. The fact that he's not in the Corps willingly is also a contributing factor. He literally believes he's incapable, despite it being shown to the audience that he isn't.
But throughout the series, he very clearly takes steps to become braver--he goes up Mt. Natagumo on his own, without anything telling him to or encouraging him to do so because he realized Nezuko could be in danger and wanted to protect her.
After Mugen Train, it's stated he's not having meltdowns quite as frequently.
He stands up to Tengen to protect Aoi and Naho--and he was initially terrified of Tengen. He joins Tanjiro and Inosuke in volunteering for the EDA mission for Aoi and Naho's sake as well.
He stands up to Daki--who he knew was an incredibly strong demon--while awake, simply because he couldn't stand how she was abusing that girl. (This seems to be the most ignored moment in his arc; it's such a good example of his growth at that point in the series and people act like it doesn't exist.)
By seasons 3/4, Zenitsu can handle solo missions. He may not like them, but he can handle them.
The letter just gave him the last push he needed--it's very realistic and very authentic, because pushing past that kind of anxiety and self-esteem issues is hard, and I appreciate the series doesn't pretend otherwise.
I really think you just have to be not paying attention to think Zenitsu hasn't grown come season 4, because he very much has.
As for Nezuko, people just refuse to engage with her in good faith.
She has far too much agency and characterization to be nothing more than a "plot device".
Nezuko is immensely strong willed. She is exceedingly kind, but also has a temper, and is even capable of (and HAS, in canon) scolded Tanjiro when he annoys her. She despises injustice, especially when it involves harm to children. She is a disciplined teenager, as her mind is depicted being able to handle being under immense strain constantly (resisting her demon instincts and resisting marechi blood). Nezuko is quietly intense. She is incredibly protective, but at the same time, likes being adored and being protected, and sees the latter as a romantic gesture. She enjoys sewing, and loves konpeito. She's good on an abacus and has managed to destroy seven abacus so far.
She also has strong battle IQ and high levels of battlefield analysis, and actively makes her own choices at several points in canon.
The main reason people seem to argue "she's a plot device" is...because she's nonverbal for most of the story.
And that is very uncomfortable.
Like there are characters that fit the concept of a plot device more--such as Sabito, Makomo, Kanae, and Senjuro (and the latter is more egregious because unlike the former three, Senjuro is still alive)--but no one ever says anything about them.
They only ever write off the nonverbal teenage girl as a "plot device".
And that...says some things.
