I went the independent contractor route and am now in the process of hiring someone, which gives me some familiarity with the additional costs you will face.
Here's what to consider:
1. Self employment tax - You now have to pay the employer side of FICA which is an additional 7.65 % (15.3%/2) that your employer previously covered up to ~118k in earnings.
2. No 401k match/pretax savings options - While you're losing the match, you now have the ability to set up a solo 401k which gives you the ability to potentially contribute much much more as you can now make your own "business contributions" to the plan. Depending on how much you're earning this may or may not make a difference in tax planning.
3. Health - the frustrating answer is it depends. Premium-wise I went from around $130/month as an employee for a good plan to $180/month for a catastrophic plan. Highly dependent on age and what your health care needs are. Good thing is health care premiums are deductible as a business expense.
4. Business insurance - Might not be necessary for you but some clients require it. I think I'm paying ~$1600 a year for a few million in coverage for general business and professional liability insurance.
5. Miscellaneous business costs - Some things add up - computer equipment or software, business costs, etc.
From a break-even stand point estimate, you probably want to tack on 50% of your hourly rate as a salaried employee. If you make $100k in salary now that's ~$50/hour. You'll want a minimum of $75/hour.
However, you should consider what the market will bear for your services and charge as much as you can that doesn't make the client think you're a crazy person. Shooting for $100/hour or greater in our hypothetical example will be a significant increase your earnings when compared to your compensation as an employee. However, you'll also want to make sure you get clients that commit to a minimum number of hours. Charging many clients $300/hour for 1-2 hour jobs is a lot harder than having one project for $125/hour for 1000 hours.
Unrelated to pricing, here are the upsides/downsides of going the independent contractor route:
1. While the work itself may be boring, psychologically, it's a lot more liberating and fun when you're on your own. You'll still have clients but no boss and that makes all the difference.
2. Changing your risk and reward profile is definitely more exciting. On my first project I made more money working 4 days per week than I did as a full time employee. However, there have been a few gaps (1 month and 3 months) in between projects where I was definitely nervous and a bit stressed. However, making more money allowed me to get through that period no problem. Plus, I now see more business opportunities. I used to think I wasn't excited about money but I definitely am now, mostly because the potential to make a lot more of it is there
3. If you'll just be a solo shop the admin side of being self-employed isn't too bad, but it's still more than you would do as an employee. Part of the boring/annoying side of running a business.
4. Most of the psychological upside of being self-employed is the subtraction of most of the negatives of working for someone else. Losing the boss, the performance reviews, the company group think, having to budget minimal PTO time, etc is a big deal. A lot of people try to self-actualize through entrepreneurship and think they'll be able to find their passion and all that. That hasn't been the case for me. The work itself is still mostly boring, but doing it on your own terms is way better.
Good luck!