Just some suggestions:
Autochannel is usually the best thing to use as it should pick the channel with the “least” interference. If the accesspoint decides to change channels, the connected equipment needs to find it again, needs time and causes potential hickups. Also check whether channel 13 is available, not all equipment supports that channel out of the box. The accesspoint on channel 13 requires that the raspberry is set to a channel 13 allowed country code.
If you do want to use a fixed channel then do a coverage check (use the analyzer on your phone) at BOTH sides of the connection. Interference and difficulties might vary greatly between sides of a house or yard. Pick a channel (preferably out of the three mentioned) which gives you the best options at both locations.
In the end: Frequencyspectrum is scarse. It needs to be shared with others. There are many more options including higher power, directional antenna’s etc., however consider if they comply with local rules and regulations.
Alternatively, consider an extra (cabled) accesspoint closer to your raspberry. Remember: Repeaters and/or extra accesspoint might might your connection better but will also cause more general interference to all. A regular repeater will duplicate your own traffic, increasing airtime by at least 100%.