Sell to Open PUT - Assignment of shares before long expiration

Will I get assigned 500 shares as soon as it reaches $60 and I keep the premium?

Most likely not. What's more likely is that if the buyer wants to close their position they'll sell the option. It's usually more profitable to sell to close a position than to exercise early. But it is possible.

Because the buyer has an opportunity to wait till Jan 23, will my investment be stuck until Jan 23 even if the stock reaches $60 way before expiration date?

No - you're not stuck. You can always "buy to close" your position. Your closing order will reduce the $9,800 "profit" you received in premium, however. If you buy the option back for more than you received for it, obviously you'll have a loss overall.

I am okay with the shares being assigned, but would not like to lose the $9800 premium.

You're not going to lose the premium. The premium is yours to keep regardless of whether the option is exercised or not. If you buy to close as discussed above, though, your net profit will be reduced by how much you bought the option for.

At expiry, however, (or if the option is exercised early) you might end up with a loss since you will have to buy the underlying stock at $60 and can only sell it for market price. The maximum loss is the strike price of the option (since you could be forced to buy a worthless stock at the strike price), but practically you can manage how much you lose.

If you want to limit your loss to less than the maximum, you can enter a limit buy order to "buy back" and close your option position for whatever you want to limit your loss to.

The main point (from a PnL standpoint) when selling cash-secured puts is that your gain is capped by what you receive upfront in premium. That only happens if your option expires out-of-the money. If your option is in-the-money, then your gain will be reduced by how much in-the-money the option is.

If you want to play around as a learning experience, that's fine, but be sure you have the proper measures in place (e.g. limit buy orders) to limit any loss to only what you can afford to lose.