Hello Suzy2014,
Thank you for the information.
Is the printer within distance to connect wirelessly to your Wi-Fi router, or can a wired (Ethernet) printer connection to one of the LAN ports be used?
As I am sure that you know, there is a distinction between AirPrint and general wireless printing. AirPrint is an Apple technology involving Bonjour/mDNS, allowing printing without additional drivers/software. This technology can be used wirelessly (Wi-Fi) or wired (Ethernet).
As you also mentioned, a Windows computer does not use AirPrint, but it can be used for wireless printing with the appropriate drivers. Is it possible to connect your Windows 11 computer wirelessly to the Wi-Fi router, or is it too far away? If yes, there are Windows 11 printer drivers from Brother, which may be useful if the HL-L2370DW printer can be connected to that same router. That could perhaps solve the PC printing problem without the need for sharing via the new iMac.
https://support.brother.com/g/b/downloadlist.aspx?c=us&lang=en&prod=hll2370dw_us&os=10068
Can the Wi-Fi router be wirelessly reached by the new iMac (and the old macOS 10.14 Mojave Mac)?
The new iMac should be able to use AirPrint (without a driver) when wirelessly connected to the Wi-Fi router (or wired to one of the LAN ports). If AirPrint does not work for some reason, you could try the Brother iPrint&Scan Mac app (albeit with limitations). Normal printer drivers are not available for this printer for macOS 14 Sonoma (the latest macOS version with Brother drivers was 10.15 Catalina; these drivers may or may not work with a few newer macOS versions).
https://support.brother.com/g/b/downloadlist.aspx?c=us&lang=en&prod=hll2370dw_us&os=10080
If you so wish, the old macOS 10.14 Mojave Mac should also be able to use AirPrint when connected to the router. Alternatively, if not, macOS 10.14 Mojave printer drivers are available from Brother.
https://support.brother.com/g/b/downloadlist.aspx?c=us&lang=en&prod=hll2370dw_us&os=10052