SCHA
Schwab U.S Small- Cap ETF
stock NYSE ETF

>VOO plus SCHA is basically a US total market approximation
Since OP seems to like Schwab branded funds for the most part, SCHB would be their version of a US total market style ETF.

It is diversified enough, but I think the bigger question is whether you actually need four funds to express that view. VOO plus SCHA is basically a US total market approximation, and SCHD is a pretty deliberate dividend tilt rather than a pure diversification move. If the goal is long-term simplicity, you could probably get very close to the same destination with fewer moving parts.

Sure. You could just use SCHB instead of VOO and SCHA if you like the Schwab stuff.

So I'd recommend ETFs. VOO is a gold standard ETF for the biggest 500 companies in the U.S. market. It's tried and true and people love it.
Do some google searches and watch some YouTube videos. Look up the S&P 500, which is what VOO covers. Look up VTI and VXUS. Look up mid caps and small caps in investing. This should help you get a baseline understanding of what you can do with your money. I can tell you exactly what I'd do with it, but I'm a random guy on the internet. Research what we suggest here so you can have peace of mind with your decisions.
That being said, I like SCHB for total U.S. market coverage, and VXUS for international market coverage. I like to have a little SCHA for more small cap exposure and a little SCHD for more exposure to non-tech sectors.
The most important thing is you're starting at 18. Just don't panic sell and you should be okay.

Yes so far SCHA doing great for me. Hopefully will continue to 2027

Research an ETF that has all of the ones you have which multiples your exposure and allows more sectors to invest in….ex. SCHA, SCHD, SCHH, SCHY…or similar products.

Switching my contributions from lump sum to DCA so I stop wondering when the best time to lump is. Although if you can agree to just lump at a regular time that's probably fine.
Also not checking my failed Intuit experiment for buying the dip. Bought at 475, just a couple shares, but then it kept going down. I stopped checking my portfolio every day and found peace. Now it's just annoying. But it's only annoying once a month. When I want a pulse on the my portfolio, I just check my main ETFs - SCHB, VXUS, SCHA, SCHD. If it's green I'm happy. If it's been read for a bit, DCA time.

I would put it in an ETF that I already own rather than an individual stock. I can't buy partial shares so I like the schwabs for their low NAV. That said, I'd put it in SCHB to increase my U.S. market exposure, SCHA for increased small cap exposure, or VXUS for a little more international exposure.