Need help optimizing prop with mercury 6hp outboard on a Highfield UL340

[CaptainRivet]

Hi all,
Would need help optimizing the prop on my mecury 6hp fourstroke outboard on my Highfield Ultralite 340. have no glue about props, so would need help here to chose the right one.
The 6hp is my backup outboard for the UL340 but weights less, has built in tank and uses much less gas then my 15hp 2-stroke so I often use it for going to shore or shopping .when nice condition and short distances.
The Ul340 is only 43kg with alubottom, the 6hp has a 7.8x8 prop.

Have the following situation:
If it flat and not much current against it easy planes with me and even another person. as soon as it does plane the UL340 speeds up nicely and keep it really plane even if some swell is approaching. From rpm at top end there is some space too, means it revs but then not get any faster, really just a bit at the top end.

But if current against or swell and I want to take off it’s short before planning but does get it done and pushing but doesn’t manage and much slower, uncomfortable and needs also more gas. So I would need more force low down to get it on plane, if less top speed I don’t care.
From diameter a 9“ prop would fit. So what would be the right prop have following choices:
8x9
7.8x8 =prop fitted
7.8x7 (org prop on the 4hp)
8.38x6
8.38x6 high trust


Yes sure that won‘t be quick as the 6hp is at bottom limit but also used for trolling by fishermen so it has significant force for a 6hp and I wanna use that to get it plane in not so good condition.

UL340 with the 15hp flies and if I need I use that, also for long distance or to diving spots. The 6hp came with my cat and is in very good condition, starts nicely and reliable, it’s the lighter 1 cylinder with internal tank so handy to quickly put on and take off. So just optimize what I have.