pi-hole can perform 2 functions - that of DNS and DHCP.
Suggest setting it to do both. That way all your traffic goes through the Pi to get DNS and is then passed along to your router. For this you would then disable the DHCP function on your router.
This is how I have my network running at home.
You don't mention which CPU you are running on that board.
It matters, because if you are running Ryzen (1st gen)/Athlon there is no support for running that bottom m.2 slot. (See page 1-14 in the mobo manual for more details -...
You can't overclock on a H110 board - the multiplier is locked.
However, you might be able to slightly adjust the HWCLOCK by a few MHz.
Might be able to overclock the RAM though.
Have you tried loading the original drivers? https://www.asus.com/Laptops/K551LN/HelpDesk_Download/
Select Windows 8 as your OS, then download the Bluetooth drivers and install that.
Since there's that surge involved, you could be looking at potentially PSU, RAM, mobo and anything else connected at the time of the storm.
Slowness could very well indicate drive issues.
Considering the usage case, the TDP difference is a non-issue as the i5 more than likely might not be used at its full potential.
The current cooling solution should also be able to handle the little extra heat without giving you any problems.