Css – How do I force R Shiny selectizeInput to fill the available space?

I’m building a shiny app that allows the user to select a number of options near the top of the page which will then affect the subset of the data that is pulled in and manipulated. Everything works just fine with the calculations. The only problem I’m having is in the display. As there are a number of options, most of which are simple radio buttons with only a few options, I like the idea of keeping the look clean by having one row of options near top of the page using a splitLayout function.

For the radio buttons this looks fine, but I also have a set of two selectizeInput boxes. These are related to each other (one is month and the other is year) so I have them in their own sub-splitLayout function within the parent row. However, when the app runs the boxes themselves are squished together and unreadable, as shown (the red circle is not part of the app):

I’ve tried various methods of adjusting the width of these boxes, but to no avail. The "Selectize Options" column itself has plenty of space, as indicated by the large amount of white space to its right before "Input 3" but I can’t get the actual drop-down boxes themselves to fill that space. If I go to the code and continually increase the cellWidth argument related to that box I can eventually get them to show correctly, but at that point I’ve pushed everything else off of the row. I can’t figure out why they won’t fit with a "reasonable" amount of space allocated to them. Hence the question: how can I force R Shiny to expand these boxes and fill up the space available to them without forcing the other row elements out of the box?

Minimum reproducible example:

library(shiny)
library(shinydashboard)

ui <- dashboardPage(
  dashboardHeader(title= "Minimum Reproducible Example"),
  
  dashboardSidebar(sidebarMenu(menuItem("Home", tabName = "Home"))),
  
  dashboardBody(
    tabItems(
      tabItem(tabName = "Home",
        h2("Homepage"),
        fluidRow(
          box(title='Options',width=12,
            splitLayout(cellWidths=c('12%','12%','25%','12%','12%','15%','12%'),
              cellArgs = list(tags$head(tags$style(HTML(".shiny-split-layout > div {overflow: visible;}")))),
              column(width=2,radioButtons('input1',label='Input 1',choices = list('A'='A','B'='B'),selected = 'A')),
              column(width=2,radioButtons("input2",label = 'Input 2',choices = list('C'='C','D'='D'),selected = 'C')),
              column(width=2,
                HTML("<b>Selectize Options:</b>"),
                splitLayout(cellWidths=c('50%','50%'),
                  selectizeInput("mth",options=list(maxOptions=12),label = 'Month',selected='01',choices = list('01','02','03','04')),
                  selectizeInput("yr",options=list(maxOptions=5),label = 'Year',selected='2023',choices = list('2019','2022','2023'))
                )
              ),
              column(width=1,radioButtons('input3',label='Input 3',choices = list('E'='E','F'='F'),selected = 'E')),
              column(width=2,radioButtons("input4",label = "Input 4",choices = list('G'='G','H'='H'),selected = 'G')),
              column(width=1,radioButtons("input5",label = "Input 5",choices = list('I'='I','J'='J'),selected = 'I')),
              column(width=1,downloadButton("download", "Download"))
            ),
            fileInput(width='100%','uploadfile',buttonLabel='Browse','Upload a File')
          )
        ) 
      )
    )
  )
)

server <- function(input, output, session) {}

shinyApp(ui, server)