The trouble is, when you dig over soil to prepare for planting, there will inevitably be weed seeds in it, and those will germinate and grow now they've been brought nearer the surface, and through the bark. This assumes you dug out any obvious weeds and their roots prior to planting the bed. The only way to stop those coming through is to use a membrane and put the bark on top, but there is a big drawback or two with that method - the membrane does stop perennial plants like your campanula from spreading/increasing sideways, and weeds will still grow over time in the bark chips - seed will blow in and germinate and grow, even with a membrane beneath.
I would just like to say one thing about your plan to make a metre wide border all the way round your garden - I wouldn't recommend it. 1 metre is not deep enough to get a proper, layered effect using shrubs perennials and bulbs. It would be preferable to create a number of much wider (or deeper, if you like, from fence to outer edge) beds or planting areas, backing onto the fence, rather than a narrow border all the way around. When you create a planting area, its best to dig it over thoroughly, making sure you extract any roots and weeds you find, then emend it with some composted animal manure or your own good garden compost, if you make it. Once the area has settled, then you can plant, and always take into account the height, and particularly the spread, of any plants you choose, and leave enough room between plants for them to develop and grow properly. This is particularly important with shrubs. The space between them may look bare initially, but you can temporarily fill that by using annuals - nasturtium seeds, for instance, popped in between will give colour during summer and leaf cover while the permanent plants settle in and grow on.
After that, I recommend no mulch for the first couple of years (unless its very dry where you are and you want to keep the soil more moist) but do recommend the use of a hoe - hoeing over regularly when any weeds which appear are still small will effectively control weed growth. That is not possible to do if you have mulch, especially bark chips or stone.
As the plants grow on and get bigger, they will eventually fill up the space available, meaning less chance for weeds to grow, but for the first 2-3 years of new beds and planting, there will be plenty of weeds trying to grow. It gets less after that because you keep destroying them with your hoe before they get a chance to flower and set seed, or spread at the root.
In response to your comments. Take a look at this image here https://www.homesandproperty.co.uk/home-garden/gardening/win-a-500-shopping-spree-at-rhs-wisley-plant-centre-32278.html, just the image, ignore the rest. Its to give you an idea of what a deep planting bed can look like. What you plant in it and how close depends on which plants you choose, because growth habits differ. A plant like a Cordyline, or a small columnar tree such as Amelanchier 'Obelisk' for instance, doesn't take up much room sideways, but grows upwards and forms a trunk, whereas a low growing shrub like a lavender doesn't get tall and needs more room sideways and all round. Your delphiniums may get a bit wider, but tend to be taller, so growing next to those you might plant, say, a Spiraea Goldflame, and dot about Euonymus fortunei varieties for their evergreen, colourful leaves, with perennials in between.So the idea is to work out a combination of plants to use, not to dig one area 5 x 5 feet and plant one shrub in the middle. Not only would that be tedious to maintain, it wouldn't even look good... I wouldn't recommend Philadelphus particularly anyway, not for a mixed smallish border or bed, but even if you did plant a single Philadelphus in a 5 x 5 bed, you'd put ground hugging/ground covering plants around it - such as Campanula portenschlagiana, Ajuga, Oregano, Helianthemum and the like, depending on sun exposure.