Use cards for variable end time
Take some playing cards and pull out the four aces. Add any 6 other cards and shuffle. Put the deck of 10 cards face down near the board.
Add 2 other cards to the top of the deck - or more for a longer time limit.
Each turn (spring is a turn, fall is a turn) flip over a card. After 4 aces come up, the game is ended.
This makes the game not have the artificial play style that happens with an anticipated end point, but still sets a time limit with some variability that creates tension after the 3rd ace is up.
Less players (or more players)
sourced from here: https://www.reddit.com/r/diplomacy/comments/119hxln/comment/j9ms6yi/ and written mostly by lambdaaurigae, though I trimmed it down a bit.
2 Players: If you want to practice your tactics, the best option is probably France vs. Austria: classic map, one player takes France, the other takes Austria. Remove the other 5 players' units from the map.
3 Players: 3P variant is 1066: https://www.vdiplomacy.com/variants.php?variantID=94
Note that this is not super well-balanced: it's generally agreed that Norway>England>France
4 Players: Thttps://www.vdiplomacy.com/variants.php?variantID=7 South America map. It's a variant of a 5 player South America map.
5 Players: South America map https://www.vdiplomacy.com/variants.php?variantID=6 .
The most popular 5P map is probably Ancient Mediterranean: https://www.vdiplomacy.com/variants.php?variantID=9 but I don't prefer it: I think there's too many centers, making it feel a little bloated compared to classic. (Also, it's not well balanced - the central powers of Greece and Egypt struggle)
6 Players: Try using the classic map, with no Italy. Keep Italy's units on the board, acting as if they are ordered to hold each turn. If they are forced to retreat, they disband, rebuilding if the center is not occupied during the winter.
8 Players: There's this map? https://www.vdiplomacy.com/variants.php?variantID=24 Never played it, and it looks kinda finicky.
9 Players: Maybe this map? https://www.vdiplomacy.com/variants.php?variantID=113 Never played it either, but it looks ok.
10 Players: I haven't played on it, but War in the Americas https://www.playdiplomacy.com/forum/viewtopic.php?f=630&t=59568 is a 10P map I've seen used, and it seems functional.