I bought lincoln cordless grease guns for my three garages at work for shop and field use, the guys love 'em..
can't speak to how leak-proof they are.. with multiple mechanics using them it's impossible to tell, lol..
getting very difficult to buy good 'ol american made tools that work and last but farm sales, auctions, etc. may land a good solid unit..
as for new brands, some google reviews like this may help,
https://toolsinsider.org/best-grease-gun-reviews/
good luck!