Hi Guerro, loving the setup you have achieved so far.
The problems you are describing I faced with my newly set up tank from last year, I knew I could line the whole thing in insulation which I have done for my hot growers but that doesn't look very good for my display terrarium so a bigger heater had to come out.
I'm surprised you are struggling with a heat cable.. but you haven't mentioned the wattage. To heat an area that is insulated probably requires 5 times less power than to heat the same area in a glass tank that is not insulated. If it is near a cold window it will need even more power. The terrarium will just constantly have to replace the heat it is losing so for your big terrarium you need roughly 50 watts of heating power.
Ok, thats fine, but then how maintain the temperature, and how about maintaining a different temperature at night time?
That's where the info kind of drops off, if you want to grow orchids in 5 degree climate's that need 20 degrees during the day and 10 degrees at night -how do you achieve that with just one set of heating elements?
I think Inkbird do do a dual day night thermostat but there was something about the functionality that I can't remember that I disliked at the time and it is also as expensive to buy as buying a second set of heating elements so I have mostly got 2 heating elements in each tank, one for day time, one for night time.
You need to make sure it can maintain the temperature you need it to. I was experiencing some issues with one of the heat mats not heating up as well anymore so needed replacing but in order to replace it I had to take out every single orchid, remove a shelf to get to the heat mat. So try to keep things as accessible and replaceable as possible.
A heat cable is good for this. I can highly recommend 30 watt wine heating cables available on ebay. They provide a good amount of heat.
Otherwise if you have the space one of my preferred ways of heating is using this
Dimplex ECOT1FT Wall-Mounted Tubular Heater 40W 408 x 81mm | Heaters | Screwfix.com
Now is a good time to buy one because come halloween over Winter they will be all sold out like last year again and then they won't get any more stock till spring because of shipping container issues.
They have a built in thermostat (not very accurate but good enough) to maintain a set temperature but you have to set it up correctly, you need to let it heat to the set temperature on max power and watch it, then at the right time when it hits the temperature you want you turn the adjustment dial down till it clicks, then it will be set to that temperature - the manual will say do not hook it up to a timer. We have to for our purposes and it is just a safety precaution on their end because it can reset the thermostat if you turn the power off but so far mine is working exactly like it should for about a year now and I just have a thermometer to check it is working as it should.
If you need more precision you can use reptile thermostats like this one
Aquarium Reptile Digital LED Temperature Controller Thermostat Control UK PLUG | eBay
For one I need to maintain a constant day and night temperature I use 2 of those thermostats. with 3 heatmats, 2 for the day, 1 at night.
I know it sounds a bit ridiculous but I literally moved a phalaenopsis from it's warm environment to my living room to enjoy a recent bloom and plop within a day the flower fell off. Took me waiting for it patiently for over a month and I had to laugh this morning looking at the wilted flower now.
So yeah phals do not like the cold and my sister who has moved abroad would not believe how cold it has gone in the middle of June here when other places are gearing up for the yearly wildfires again and we either can't grow them or we need to try to keep them happy.
It is a bit excessive but it only seems that way because it is a plant, tropical fish and other exotic pets needs to be heated too.
So yeah I know how little info there is out there and some of my phals closest to the glass did struggle still so I am hoping having replaced my heat mat things will go better this winter.