Spoiler about her possible marriage

This is long but so far is what the author has set in the story, so is not groundless.
If you want to read the novel, here's the link: https://ncode.syosetu.com/n8406bm/183/

So let's start.
Shizuko doesn't want to get married, she can't get married and won't get married.
Why? So far in the novel, which spams as far as 12 to 13 years, this is explained, so here comes the reasons.
Shizuko in modern times was raised by her grandfather to be the next head of the family, so he made sure that she had tons of knowledge regarding how to organize a business and how to maximize the potential in people. With this knowledge, he made sure she would ok no matter what, so what she needed was now someone to be next to her and help her grow and support her. Since she is still so young (remember she was 15 when she isekaied) her family (with granpa at head) where looking for someone to match her and develop feelings. Shizuko as far as has been show she is very understanding and accept things as long as it doesn't affect her in any negative way, so when this part was explained she said, that sure, she talked about love with her friends but she didn't go beyond that, so when her family talked about this, she was even happy and ok with it, because she realized that her family wasn't trying to control her, but find the best person out there for her, meaning they were doing it out of love, because they knew she wouldn't even think about it. So Shizuko it's ok with an arranged marriage because the other person is someone she can grow to love.
This explain why she doesn't want to get married.
Now let's move to the sengoku period.
As show in the manga and novel, Shizuko, it's part of a prophecie about a bastard time, so that explain why Oda accepted so quickly and gave her a villa. Shizuko herself had this dream of using her modern knowledge in the sengoku period, so this more or less freedom of action let her become filthy rich, like she could topple Nobunaga if she wanted.
Through the novel, step by step is explained how she improves from land to infrastructure, going through all industries thought, making sure that the moment the industries are developed she records all the information and gives to the warlord under Nobunaga, making sure that the technology is spread but she still get huge benefits from it, which she invest again in another industries, so creating a virtuous cycle. At the same time she nurtured the Oda army and created and elite army of her one, not for personal use, but to the Oda army when they need it for special situations.
Like this the people living in her territory and the army are so loyal to her, to the point when they find spies, the first reaction is to show them how good life is there (which made many then around and live there), but if this didn't work they would report them or even beat them. Even if the warlord tried something at her (wether Oda vassals or enemies) they couldn't do anything because her uncle did all the dirty work to make sure she would be safe, but only the desperate ones do this, because if help is needed, Shizuko will find and Oda inform him, and make a plan to help them. Even one of the vassal under a warlord asking to borrow money from her for they lord, got surprised that she even doubled the amount, an amount that his lord wouldn't even have, while she has it in one of her warehouse.
So with this explanation, we get the gist of it, Shizuko is filthy rich, remember she is the adopted daughter of a very prestigious family close to the court, a super loyal follower of Oda and someone who will help people in need.
Even though she is the perfect person to marry, the moment she does, she will stop being her solo persona and will belong to her husband family, meaning that she won't be only to get them, but she has to strengthen her husband family. Even someone so desesparate though of marrying her to kill her to get all the benefits. Even if Oda ever gets the idea of getting her married (he talked about this with her uncle, the old man the uncle isekaied and her adopted father), he won't do it because if he allows it, she will have to be loyal to her husband family and if they break from Oda, she will have to follow her husband.
So Shizuko can't get married.
Now, why she won't get married?
So far we talked about all the benefits, but what will happen when she dies? Here come Oda again.
In feudal japan there was the belief that twins were bad luck, so when a Oda concubine gave birth to a pair of twins (boy and girl), he didn't kill them (Oda loves his children), but regretfully didn't have them enough attention, so they were mistreated behind his back. When he found out he was furious so he jailed the nanny in charge of them, and made Shizuko adopt them, this way protecting them from the possible future mistreatment because of the superstition and strengthening his connection with Shizuko, sending this way many waves along the way.
Many though that this way Oda was going to start a hereditary war for the head position taking it away from Nobutada, and wanted to take advantage of the opportunity. But the boy made his point clear, he was Shizuko's son and future heir, meaning that although he had tides with Oda his loyalty was with Shizuko, and through her he had loyalty to Oda but as a vassal.
Shizuko won't get married because now she has a heir that will protect all she has done during her life.
This is based on the novel and so far even if the author wanted to marry her, it would be difficult, beside all the explained because now Shizuko is old, not to modern people but to the Sengoku standard, as they marry when 13 or less.
And before you ask, no don't even think about Tadakatsu the guy with the spear, because he belongs to Tokugawa, so there's a snowball in hell Oda will allow it, no matter the merits he gets (Tadakatsu thought that if he got enough military merits, he will catch her attention).