I do a lot of PDF work which requires edits, encryption, etc. Unfortunately the only solution I found that worked for me was a paid one. I use Code Industry's Master PDF Editor.

This isn’t a gui solution and probably is way off from your intentions, but you could use pdftk to dump the fillable field name and pdftk again to populate them with an xfdf file.

edit, I got interested:

#!/bin/bash

# Check for required arguments
if [ $# -ne 2 ]; then
    echo "Usage: $0 <source-pdf> <destination-pdf>"
    exit 1
fi

SOURCE_PDF=$1
DEST_PDF=$2
XFDF_FILE=tmp.xfdf

# Extract form field names
FIELD_NAMES=$(pdftk "$SOURCE_PDF" dump_data_fields | grep "FieldName:" | cut -d ' ' -f2-)

# Start XFDF file
echo '<?xml version="1.0" encoding="UTF-8"?>' > $XFDF_FILE
echo '<xfdf xmlns="http://ns.adobe.com/xfdf/">' >> $XFDF_FILE
echo '<fields>' >> $XFDF_FILE

# Prompt for user input for each field
for FIELD in $FIELD_NAMES; do
    read -p "$FIELD: " INPUT
    echo "<field name=\"$FIELD\"><value>$INPUT</value></field>" >> $XFDF_FILE
done

# Close XFDF file
echo '</fields>' >> $XFDF_FILE
echo '</xfdf>' >> $XFDF_FILE

# Populate the PDF form
pdftk "$SOURCE_PDF" fill_form $XFDF_FILE output "$DEST_PDF"

# Clean up
rm $XFDF_FILE

echo "PDF form filled and saved as $DEST_PDF"

Either firefox or libreoffice draw

I've been looking for a decent PDF editor on Linux for years. Like you said, there are plenty that will basically work, but I always have issues with font mishandling.

So far I've just settled on using a windows VM with adobe for editing PDFs (along with one other windows only program that I need.) There is a way to get Adobe PDF software working in linux, but I haven't tried it.

If you need to sign PDFs, xournal++ is an excellent app for applying a saved signature as a stamp.

I just screenshot the PDF in fullscreen and then use kolour paint to add in text, it's worked well for me.

Foxit still provides a free version that's linux compatible. Its been a lifesaver at work to do document signing without messing everything up. It may take a little tweaking to run, but it's worth a try for forms.