Ive seen those little "transmitters" on ebay, they work OK as you have seen, but not very well. They are just 3 crystal oscillators and a transformer, 3 for the different frequencies. Its about as simple as you can make yourself but they wont produce great audio like a better device would.
In terms of value, you would struggle to beat this device:
https://www.nfor.nl/radioforumservic...PLL_AM_Osc.pdf
Its IC based and uses a 4053 analog switch IC as the final. Very stable and running at 15v could well cover your whole house properly with the right antenna and other variables. It gets better range than my SSTran AMT3000 did
A kit for building this circuit can be bought here for £15-£18
https://www.amateurradioshop.nl/
You need to use google translate though as its in dutch, I've found them very helpful by email as well. There are many different transmitters available, if you don't fancy building one the spitfire is ok, but I suspect the one I posted above is better.