Are you keeping the combi for hot water?
If it were me I’d rather have an ASHP running at 75% capacity by having a bigger unit than a smaller one chuffing it’s nuts off.
Vaillant now do an all in one unit with nearly no plumbing with cylinder, expansion vessel etc in a unit that looks like a fridge freezer plus the heat pump outdoors then you have hot water and heating all in one tidy unit it doesn’t even need an immersion cycle to prevent legionnaires, if you wanted to switch completely. Ours is the older school cylinder expansion vessel all plumbed separately and they took an age to do all the plumbing by hand the unitower looks much better.
Seeing as the guts of the system will be with you for decades I would go for the larger unit you have the option of adding radiators later that way too.
After initial teething issues gotta say it runs well, keeps the house warm all the time, there’s no gets really hot then too cold, if you have a weather compensating add on that helps too and our electricity bill is quite reasonable with Agile and having the heating shut off between 4 and 7pm each day to avoid peak kwh charges.
We keep ours down to about 15p/kwh and about £130 a month which includes paying for the extra winter usage