Merge of logs from old and new RPi servers is simply a concatenation.

% gzip -d < 202008-old.json.gz | tail
[ "2020-08-21T20:05:34Z", "", {"@":"95ABDC89C1999C9E","+":14,"err":20,"T|C16":356} ]
[ "2020-08-21T20:06:27Z", "", {"@":"E091B7DC8FEDC7A9","+":10,"L":2,"T|C16":381,"H|%":54} ]
[ "2020-08-21T20:06:28Z", "", {"@":"96F0CED3B4E690E8","+":13,"L":54,"tT|C":17,"tS|C":0} ]
[ "2020-08-21T20:06:41Z", "", {"@":"FA97A8A7B7D2D3B6","+":2,"R":245,"err":0,"T|C16":364} ]
[ "2020-08-21T20:07:12Z", "", {"@":"91ACF3CFF388D4E0","+":6,"vac|h":10,"L":1,"v|%":0} ]
[ "2020-08-21T20:07:29Z", "", {"@":"FEDA88A08188E083","+":6,"T|C16":387,"H|%":54,"O":1} ]
[ "2020-08-21T20:07:36Z", "", {"@":"95ABDC89C1999C9E","+":15,"H|%":57,"O":2,"vac|h":0} ]
[ "2020-08-21T20:08:28Z", "", {"@":"96F0CED3B4E690E8","+":14,"vC|%":21,"gE":0,"R":34} ]
[ "2020-08-21T20:08:33Z", "", {"@":"E091B7DC8FEDC7A9","+":11,"O":2,"vac|h":0,"B|cV":290} ]
[ "2020-08-21T20:08:41Z", "", {"@":"FA97A8A7B7D2D3B6","+":3,"O":2,"H|%":56,"vac|h":0} ]
% gzip -d < 202008-new.json.gz | head
[ "2020-08-21T21:12:25Z", "", {"@":"96F0CED3B4E690E8","+":14,"tS|C":0,"vC|%":21,"gE":0} ]
[ "2020-08-21T21:12:35Z", "", {"@":"E091B7DC8FEDC7A9","+":11,"L":27,"O":1,"vac|h":0} ]
[ "2020-08-21T21:12:39Z", "", {"@":"FA97A8A7B7D2D3B6","+":3,"T|C16":361,"vC|%":629,"O":2} ]
[ "2020-08-21T21:22:23Z", "", {"@":"96F0CED3B4E690E8","+":3,"gE":0,"R":34,"err":0} ]
[ "2020-08-21T21:22:25Z", "", {"@":"E091B7DC8FEDC7A9","+":0,"L":26,"v|%":0,"tT|C":6} ]
[ "2020-08-21T21:22:33Z", "", {"@":"FA97A8A7B7D2D3B6","+":8,"err":0,"T|C16":361,"H|%":58} ]
[ "2020-08-21T21:23:18Z", "", {"@":"91ACF3CFF388D4E0","+":12,"tT|C":12,"tS|C":3,"vC|%":49} ]
[ "2020-08-21T21:23:35Z", "", {"@":"FEDA88A08188E083","+":12,"L":179,"T|C16":388,"H|%":56} ]
[ "2020-08-21T21:23:40Z", "", {"@":"95ABDC89C1999C9E","+":5,"H|%":59,"O":1,"vac|h":0} ]
[ "2020-08-21T21:24:20Z", "", {"@":"96F0CED3B4E690E8","+":4,"T|C16":353,"H|%":59,"O":2} ]
% cat 202008-old.json.gz 202008-new.json.gz | gzip -d > 202008.json


For the record, the merge to make the annual file is:

% cat 2020??.json.gz | gzip -d | xz -v --best > 2020.json.xz