I'll set up WIFI in a coffee shop,
The environment there is very noisy,
In the store, you can search for a large number of SSID from the surrounding stores.
If I use SXT SA5 AC as an indoor access point, will it be appropriate? (reduce power usage)
I think its directional antennas can reduce interference,
Better delineation of each AP range.
Are there any inappropriate places for outdoor AP to be used indoors? Or is WAP AC more appropriate?
(there are lots of guests playing games in the coffee room. They are very demanding for WIFI)

As long as you do not need any 2.4Ghz you should do fine. With traditional access points you try to place near the users for the best performance. This would be more suited for a corner mount that's facing the customers. If you need dual band the hAP AC is pretty awesome.

As long as you do not need any 2.4Ghz you should do fine. With traditional access points you try to place near the users for the best performance. This would be more suited for a corner mount that's facing the customers. If you need dual band the hAP AC is pretty awesome.

200 square, open space, 100 people at the same time play the game
I plan to arrange 5 AP (5G only), channel (20MHZ): 36,48,149,157,165.
Because the game player on the network quality has a high demand, so each channel interval 20MHZ, to prevent adjacent channel interference.

Who can give me some more professional advice?

You should take care with the tx-power of SXT because this CPE have 1000mW of power and in a indoor enviroment this may be a big problem because CPE may causes interferences itself.

You can use this device but you need to set tx-power to a few dBm's to avoid saturation of clients and interference problems.

Regards.