A possible solution is to purchase a convertible 2-in-1 laptop and a monitor with adjustable height. While at home use the laptop with the screen laying flat on the table and connect the monitor to it. If you fold the screen around on a 2-in-1 laptop so that it looks like a large tablet, the keyboard will be disabled. If you need to use the laptop away from home, then simply disconnect the monitor.
An example of a convertible 2-in-1 laptop is the following 15.6" Lenovo Flex 5 which has a Core i5-7200u CPU and 1080p screen for $700. It can be upgraded to 16GB of RAM a link to video demonstrates how to do it below.
An alternative to a ITX build is an Intel NUC build.
An Intel NUC is basically a small barebones case that has a motherboard that uses a mobile Intel CPU. All you need to do is add RAM and a M.2 SSD; taller versions will also have a 2.5" bay for a HDD or SSD. The NUC can be mounted on the back of a monitor as long as the monitor is VESA compliant. Using a bluetooth keyboard and mouse basically means there the entire setup will only have 3 cables. One cable to power the monitor, one for the NUC and one to connect the NUC to the monitor. With the right cable management, this can be a very "clean" setup.
RAM does not generate heat especially when it only uses 1.2v of electricity. If you use a laptop or computer for a long time turn turn off and immediate touch the RAM sticks at worse it may feel warm. Heat is primarily generated by the CPU and GPU (if there is one). Overheating could be caused by a poorly designed laptop with insufficient cooling and / or airflow.
If you an ITX can fulfill your needs, then there is really no need for you to consider a laptop.
There could be a number of things that can make a laptop or desktop "feel" slow. That includes background processes running in the background or launching a program from a hard drive instead of a SSD. It could be the programs themselves. The CPU also down clocks itself when there is minimal usage which is a built in function of Intel CPUs known as SpeedStep. AMD's APUs and CPUs do the same thing and it is called PowerNow.