In consideration of Germany’s Hs 129 B-3 with its 75 mm BK 7.5 cannon, I would like to suggest a Japanese analogue for the sake of content, fun and variety at the same BR rating of 1; it requires equal precision and all the same downsides of such a plane with a considerably large gun of limited ammo capacity.
The Mitsubishi Ki-109 Heavy Fighter/Interceptor.
It was a modification of the Ki-67 Bomber, designed with the purpose of taking down American B-29 planes with it’s 75mm Type 88 anti-aircraft cannon mounted in the nose, with an ammo capacity of 15 rounds.
The design was approved in 1944 with 2 prototypes developed in the same year. By March 1945 twenty-two Ki-109 planes had been produced and delivered to the 107th Sentai unit with the purpose of protecting the mainland.
The ammunition used in the cannon would be the Type 95 - 7-cm (75-mm) Armor-Piercing High-Explosive Projectile. In terms of its penetrative performance, it was designed to penetrate 20mm of steel at 3,000 metres when used in the Type 41 Mountain Gun. So it would be reasonable to assume it could reliably penetrate around the high twenties and low thirties with the longer barrel of the Type 88 AA gun mounted in the Ki-109. Enough to take out any Allied tank with some well placed shots above the engine bay and directly on top the tank, with some precisely placed shots.
Additionally even when not attacking armour, the large amount of explosive filler will splash infantry and lightly armoured APCs quite well.
I hope you like my suggestion and let me know what you think. I believe it could be a fun little option for Japan
Here’s an additional photo of the 75mm Type 88 AA gun and a diagram of the Type 95 - 7-cm (75-mm) Armor-Piercing High-Explosive Projectile that could be used. There’s also a HE she’ll with higher filler if a mix of both APHE and HE would be better.
Some sources
Ki-109 Hiryu Mitsubishi
TM 9-1985-5; Type 95 7-cm (75-mm) Armor-Piercing High-Explosive Projectile
Japanese Explosive Ordnance – Army Ammunition – Navy Ammunition Chapter 4 – Section 1