Have you checked your video settings on the P4P? You may be able to dial down on the streaming bit rate.
I have a K1 and it seems to have a decent amount of firepower. It was designed and marketed as part of a gaming system and as such can handle the video stream from both my Phantoms 3 S and P. Although no longer in production, used K1's in very good condition are frequently offered on e-Bay and Amazon. It's also the only tablet I know with HDMI output that connect to my goggles.
You really might need a newer iPad to be able to handle the high quality from the lightbridge 2 connection that your Phantom 4 gives. Go into lightbridge settings and see if you can turn down the quality in the app. Also, make sure you are using a high-speed data cable, not just a charging cable. I used the charging cable that came with my Huawei phone when I got my Phantom 4, and I thought my new Huawei phone was not powerful enough, because the video was very glitchy. I doubted that it was my expensive flagship phones fault, so I went and bought an Anker powerline USB c cable. It was $20, kind of expensive for a cable, but it solved all my issues, I have perfect video signal now. So maybe try getting a high-speed lightning cable for your iPad to see if that works, try that before spending a ton of money on a new tablet.
Anker Powerline+ II Lightning Cable (3ft), MFi Certified for Flawless Compatibility with iPhone Xs/XS Max/XR/X / 8/8 Plus / 7/7 Plus / 6/6 Plus / 5 / 5S and More(Black) Amazon.com: Anker Powerline+ II Lightning Cable (3ft), MFi Certified for Flawless Compatibility with iPhone Xs/XS Max/XR/X / 8/8 Plus / 7/7 Plus / 6/6 Plus / 5 / 5S and More(Black): Cell Phones & Accessories
