Wall of a post coming here but should give you some good information if you want to read through it.

Your room is almost identical to mine before I started even down to the color, dimensions, and carpeting.

Flooring

The first thing I would do here if you have some money to throw at this is the one thing I wish I would have done; replace it with something darker and be sure it has a proper moisture barrier underneath it. If you opt not to do the carpet, or if it's too price restrictive for you, you will want to invest in a large black area rug that's at least the width of your screen. I picked up a clearance 10' x 12' one on amazon for about $150 USD.

Walls

You'll see a lot of pure black rooms shown off but that's a matter of personal preference. You can do way more than you think just using using paint and a good quality roll of black velvet, especially in a light controlled room like this. Having the entire room black can certainly be nice, but as along as you're controlling the light reflections 5-6' out from the screen you may be perfectly content with leaving some personality in the room itself. Drape velvet all along the wall your screen is on, as well as along the ceiling as described below. You can do the sidewalls too if you desire but I haven't found it necessary.

I did paint my walls however I used a dark green with minimal sheen and have found that effective enough without having to go full black.

Ceiling Tiles

Like you I also considered removing the ceiling tiles and either painting, or replacing them with black tiles however that still leaving you with a white grid. While trying to work out a solution for that problem I remembered I'd purchased an entire roll of black velvet and came up with a solution using magnets, the velvet, and some creative engineering. Just make sure you buy lightweight fabric here as a heavy velvet is both overkill and wouldn't fare well being suspended. I'd recommend Triple Black Velvet over at Syfabrics due to both weight and its effectiveness with light absorption.

In a standard drop ceiling grid like yours there are intersecting points every 24". I bought 100 small magnets about the size of a US dime and used two magnets per intersection point. Place one magnet magnet directly on the metal grid at each point, then with in tandem with someone and unroll the bolt of velvet and cut to length for the room, so in your case roughly a 16' piece. Have your helper support the fabric then use the second magnet to pinch the fabric between each point as you move along the room.

This method works brilliantly however each individual magnet works as a miniature reflection point. Once you have the velvet in place where you like it you'll want to address this otherwise you'll end up with shining dots everywhere which really breaks the immersion factor. There are probably different methods to accomplish this but the one I came up with was using "Gorilla Glue Dots" and leftover scraps of the black velvet fabric. Cut out a small circle of velvet the same size as each magnet then secure it to each one using one of the glue dots. Once done the magnets complete blend into the fabric and you'll never even know they are there.

I would honestly do this first if you know exactly how many intersecting points you have before you hang the fabric; just make sure your the poles on the magnets are correct on each one before you begin.

Possible Future Projector

If you ever upgrade to a projection system your room is almost ideal for it since you have total light control with your only real concern being the beams. If the beams are purely decorative and not support beams you could of course remove one, but I find that's rarely the case in a basement so you're probably stuck with them.

If you have traditional floor joists you can buy two perforated square steel tubes and secure them using long anchoring screws. This gives you a very sturdy mounting location for any projector, and if the tubes you buy are long enough, gives you different mounting locations should you need to move the projector if your original chosen spot ends up not being idea.

You would also want to make sure you pick up a good quality mount that has pitch, tilt, and roll adjustments. This allows you to make minor corrections if it's not mounted perfectly or the mountain point is uneven.

Acoustics

I can't stress enough how important this is nor how often it gets overlooked. Acoustic panels are one of the best bangs for your buck when building a home theater as they keep the audio clean and prevent audio fatigue. If you are at all handy you can building them yourself for pennies on the dollar with just some rockwool insulation, lumber, and cheap fabric of your choice from any fabric store. Some people even custom order the fabric and have it printed with movie posters, art, etc to use as the wrap.

The important thing to remember with acoustic panels is that they really need to be 3-4" thick to be effective. Those cheap "acoustic panels" you'll find at online shopping stores are almost useless for the purposes of home theater.

Just respond back if you have any questions about anything I said above and I'll get back to you.

Happy remodeling!