The Wiki shows how you can do this: https://github.com/fastfetch-cli/fastfetch/wiki/Format-String-Guide

if you run this, it tells you what the format options are:

PS> fastfetch.exe -h uptime-format

-- In config file: { "type": "uptime", "format": "{<format-variable>}" }
Sets the format string for Uptime output.
To see how a format string is constructed, take a look at https://github.com/fastfetch-cli/fastfetch/wiki/Format-String-Guide.
The following variables are passed:
              {days}: Days after boot
             {hours}: Hours after boot
           {minutes}: Minutes after boot
           {seconds}: Seconds after boot
      {milliseconds}: Milliseconds after boot
         {boot-time}: Boot time in local timezone
             {years}: Years integer after boot
      {days-of-year}: Days of year after boot
    {years-fraction}: Years fraction after boot
         {formatted}: Formatted uptime

I've just updated mine with the below to demonstrate:

        {
            "type": "uptime",
            "key": "{#magenta}╰─ {icon}  {$1}Uptime{$2}",
            "format": "{days} days {hours} hours {minutes} minutes"
        },

This is what it looks like:

Just put the variables you want in curly brackets and normal text outside (but within the quoted format string).
e.g., if you just put the format string as "{days} days {hours} hours {minutes} minutes", the output would simply be (in this example) "2 2 30".

This applies to a lot of the modules, so you can customise how the output is displayed using that "format" line.

Hopefully that makes sense.

Yes I've seen the formatting options, problem is that this doesn't account for singular units, so it will also say "1 days 1 hours 1 minutes". I'm guessing there's no way to account for that?

My apologies; I misunderstood your query.

I don't think you can do that with the built-in uptime module, however you could bypass that and use a command module instead.
I've just tested this on one of my Linux machines, but it should also work on MacOS. For windows you'll have to see if you can find a similar way to do the string manipulation:

// Custom Uptime Command
        {
            "type": "command",
            "key": "{#magenta}╰─ {icon}  {$1}Uptime{$2}",
            "text": "awk '{d=int($1/86400); h=int(($1%86400)/3600); m=int(($1%3600)/60); s=int($1%60); printf \"%d %s %d %s %d %s %d %s\\n\", d,(d==1?\"day\":\"days\"),h,(h==1?\"hour\":\"hours\"),m,(m==1?\"minute\":\"minutes\"),s,(s==1?\"second\":\"seconds\")}' /proc/uptime",
            "format": "{1}"
        },
// Test Uptime Command (1 day 2 hours 3 minutes 1 second)
        {
            "type": "command",
            "key": "{#magenta}╰─ {icon}  {$1}Uptime{$2}",
            "text": "echo 93781 | awk '{d=int($1/86400); h=int(($1%86400)/3600); m=int(($1%3600)/60); s=int($1%60); printf \"%d %s %d %s %d %s %d %s\\n\", d,(d==1?\"day\":\"days\"),h,(h==1?\"hour\":\"hours\"),m,(m==1?\"minute\":\"minutes\"),s,(s==1?\"second\":\"seconds\")}'",
            "format": "{1}"
        },

note: this is a bit rough as it was a quick text; the icon is not specifically declared so it just shows the default for the command module.

Two versions here; the top one is what you want, the second is just me forcing a specific time to prove it works.

Essentially, the command gets the uptime from /proc/uptime in seconds, manipulates the output to set day, hour, minute and second values, then finally applies a conditional test to set the text after the number; i.e. if the value is 1 then it is day/hour/minute/second, otherwise it is days/hours/minutes/seconds.

The second version sets a fixed time and drops the passthough to /proc/uptime to prove the conditionals are working.

Someone else might be able to confirm if you can do this with the native uptime module, but in the meantime this should give you what you want.
Screenshot; top uptime is my actual uptime, bottom uptime is the forced example.

I find these custom modules super useful and very flexible. I've used them for simple things like "oh-my-posh --version", up to a relatively complex awk command to extract the status of a MDADM mirrored volume.

edit: I asked a popular AI model so take this with a pinch of salt, but it claims that this command will achieve the same thing in Windows:

        {
            "type": "command",
            "key": "{#magenta}╰─ {icon}  {$1}Uptime{$2}",
            "text": "powershell -NoProfile -Command \"$u=(Get-Date)-(gcim Win32_OperatingSystem).LastBootUpTime; $d=$u.Days; $h=$u.Hours; $m=$u.Minutes; $s=$u.Seconds; \\\"$d $(if($d-eq 1){'day'}else{'days'}) $h $(if($h-eq 1){'hour'}else{'hours'}) $m $(if($m-eq 1){'minute'}else{'minutes'}) $s $(if($s-eq 1){'second'}else{'seconds'})\\\"\"",
            "format": "{1}"
        },

I ran the un-escaped command in a PowerShell session and it appears to work.

edit2: i just noticed that my uptime is completely different from the screenshot in my first reply; a different machine obviously but I confused myself for a moment there 🤣