Questions about repairing iPhones.

Hello Hello.. I have a question for those who do iPhone repairs quite often or even for a living.

basically, I learned how to replace an iPhone 6 screen quite awhile ago, I haven't done much with the knowledge except buy a couple cracked phones on eBay, let go, and offer up and replace the screen with a brand new one and sell it making 60$ - 100$

The only phones I've technically repaired are just regular 6gs and 6+'s. I like doing it and all but I want to get better and there's a lot of ups and downs to buying cracked phones and selling them online. It just takes a long time to find a buyer mostly, and sometimes I'll buy a phone with a bad ESN or something. Happen once but I have learned my lesson on that.

I've technically never replaced a 6s or 6s+, I know breaking the seal when opening the phone makes it a little more difficult but for the most part it's not much harder than a regular 6, from what it seems like.

I currently work at an iPhone refurbishing business so I have a lot of resources and the ability to get screens at the place I work. Which is why I want to possibly transfer from being someone who just buys and sells phones to someone who repairs on the go for people, like a mobile tech repair.

My questions are, how did you branch off from just doing casual repairs to being able to post an ad on an app and start doing repairs quite often for people?

How many times should I do a repair on a phone model before feeling comfortable to do it for potential customers, rather than just friends and on phones I buy myself.

As there are quite a lot of mobile repair services, how do you go about heating the edges of the 6s and 6s+'s to loosen the adhesive? like if you were doing a repair at a Starbucks (for example) would you find some way to power a heat gun?

Or after doing 6s's for quite some time, do you not even need the heat? I know it's not mandatory to take off the screen but I'm positive it will make it a bit easier for someone's who is not an expert yet.

Do you think to further my skill I should just buy more cracked phones, maybe a 6s and then a 6s+ now, and just try the repair on those. Can always resell them as well, what worked for you when you were first starting out? Maybe I can just buy an icloud locked 6s and 6s+ for cheap and perform the repair, and then just donate it to my work as a tester. So no major loss really but some money and I'll know I can do a 6s repair at least. Thanks, sorry for the extremely long question!