What are some things I could do to be prepared early for my future ETS to be ahead for a smooth transition?

Get copies of everything in your medical record and make sure any problem you have is documented.

First, figure out what you want to do for money afterwards. Everything else is simple. Once you know where you want to be in five years, it's just backwards planning. If you're a year out and you don't know yet, extend another year and revamp your plan.

Also, pay off all your debts before you separate and save a few thousand for the inevitable break that will usually come in your employment. Most people don't walk directly into a job, so be prepared in case that's you.

Don't count on BAH from the VA as a solid income source when you're budgeting. They're known for being late and you only get full BAH when you're in school full time for that whole month.

Continue to establish continuity of care for any medical conditions. Review CFR 38 Part 4 for any medical conditions to ensure you properly articulate your symptoms.
For Resumes: obtain electronic copies of duties and responsibilities for your positions.
Create a Word document and copy and paste the description of every course you've attended (Army Formal Schools Catalog), college/university catalog course description, and syllabi. Use these for resume building. It'll save time trying to best capture your skills, knowledge, and abilities.
Volunteer. Resume, and applications for leadership training/developmental programs. Chaplain, and other agencies provide opportunities. Don't limit yourself to post.
Be prepared to apply for 50 jobs, and to manage which version of your resume you used for specific jobs (I added the announcement number to the resume file name, and tailored every resume for rhe specific job.
Research, practice, and develop your interview skills.
Learn how to dress like a civilian. Purchase, and tailor clothes for positions you seek.
If you're staying in your field, take advantage of certifications through Army programs.
Find a way to crossover all of your knowledge, skills, and abilities. (e.g. I was CBRN= Sr Defense Analyst, Emergency Manager, Continuity of Operations (COOP) program manager Safety Specialist, Emergency Operations Center (EOC) staff analyst.
Take every FEMA, NIMS/ICS, and COOP course you can. Apply for certificate available for completion of a specific series of courses.
Be overqualified.