pros and cons of sending your design to a company and letting them heat press your design on shirts rather than buying your very own heat press and getting your designs put on that trans paper to be pressed yourself. Which is more cost effective?
If you are sending a design out to be printed on demand, it's probably not going to be heat transferred. Most likely it will be printed with a DTG (direct to garment printer)
If you have the time and money to buy the equipment and learn the learning curve of making your own t-shirts yourself with a heat press, I say go for it.
Just remember that there are more than one type of "heat transfer" and not all of them are the same quality.
Inkjet heat transfers are ones that you can print at home on a desktop inkjet printer, but the quality isn't as high as other types of heat transfers.
Plastisol transfers are when you send a company your design file and they screen print your design (using screen printing inks) onto transfer paper, and send you back the printed sheets of transfer paper with your design on it. You can then press the designs onto the t-shirts as you need them if you have a heat press. The quality of plastisol transfers is very high, and if done right, probably couldn't be differentiated from a t-shirt that was screen printed directly with plastisol ink.
There are also
vinyl transfers that are cut with another piece of equipment (a vinyl color) This is good for single color designs, and if you invest in a vinyl cutter, not only can you do t-shirts, you can also do decals for cars, boats, laptops, etc. T-Shirt vinyl is very durable and can have a very soft finish.
But like you said, it's great if you're starting out, but if you get really busy (or want to go on vacation), do you really want to be stuck with having to press and ship out 50 t-shirts a day?
If it gets to that point, then that's a good problem to have

That means you're probably making enough to think about other production options. Outsourcing the printing to a screen printer could give you higher profit margins and once you've figure out which designs sell and which sizes, you'll have a better handle on inventory needs.