I have tied people up and made photos/videos of them and been paid for it ... does that count as being a professional?

TL;DR: It's something to consider a lot about

Sorry, I have a habit of writing a whole bunch about certain things. This might be edited down to much shorter as I think about it more.
Now for the more serious stuff:
I have been approached by several popular bondage sites that you might have heard of about doing a few shoots in the past with them. It definitely is something that shouldn't be accepted without some consideration.
Firstly and most importantly, why are you considering doing bondage modelling? Are you seriously wanting to get into public bondage modelling on the internet? Are you a novice who wants to get tied up and consider working with a site like TieGuyUK to be a safer way to play? Do you want to get tied up by someone who knows what they are doing? Are you a fan of TieGuyUK's work and think their stuff looks fun to try? I'm not sure how TieGuyUK manages their shoots and if they are planned or totally free for all. They are a business and I am sure they know the content their members are looking for. From personal experience both as the sub and the dom, bondage for more specific videos that are for money is much more different than just general messing-around-with-friends bondage. For me, it's a job, not play time as time is money and any slight deviation from what was planned can sometimes mean a complete restart. That dynamic doesn't exist in "normal" bondage that probably exists in your fantasies or if you casually played with someone. If you have never done professional work and worked on time schedules and never done bondage before, it may be a bit intimating to tackle something like that for your first time. But again, I have no idea about TieGuyUK's shoot structure, though I'm sure they have worked with total novices before.
Are you okay with your little hobby being as public as it could be from a site like TieGuyUK? I do barely anything public and it has at times become rather overwhelming for me with the amount of public exposure I get and I have no control over it. That certainly could become a problem down the line.
While I believe TieGuyUK is more centered around softcore and non-sexual bondage, there are some people who see it as a form of pornography. Have you thought about you being perceived as a pornstar to some people on the internet?
Obviously TieGuyUK and similar sites have a business to operate. Are you okay with being someone's paycheck? Maybe that's not the proper way to say that so I'm sorry. The places that have contacted me often make quite a lot of money off of me and give me a small amount. As a not-so-unrealistic example, one wanted to do about four hours of video with me for two days which takes let's say eight hours to film with all the behind the scenes work. They wanted to pay me 400$ which to some people sounds awesome, I mean, it's like 50$ an hour, even more if you get done faster. From that four hours of footage, it would have been split into ten or more videos for sale at a price of 20$-30$ along with several photos too available to paying members. Based on statistics for their other videos spread across several sites, each video gets conservatively 50 sells. 50 sells * 10 videos * 20$ per video = 10,000$ (ignoring the photos members get to see) of which I got paid 400$. Just something else to consider.
For TieGuyUK, you get a six month membership to the site ($70), travel expenses, and "a nice afternoon of bondage." From personal experience, that last benefit is priceless and awesome. I have no idea what TieGuyUK's membership statistics are nor their expected income gained by having you as a model.
These are just some various questions that I have had to consider personally because I have been in a similar situation as you, except studios sought me out rather than me seeking them out. I can talk a lot more about any of those questions and sorta picked more on the last one as money seems to rule the world haha.
You say you have been really thinking about it lately so I hope you have considered the above questions as well as many others. There certainly are pros and cons to doing modelling for a professional site such as TieGuyUK. Out of the studios I have seen, TIeGuyUK is at the top currently of my list of "if I HAD to work with a modelling site, here is who I'd pick"