You can use buttercream on outside or use chocolate ganache on outside to avoid the cake being too sweet. You can also experiment wit different version of buttercream, for example I find the version with egg yolks (similar to one here, https://www.notyourmotherscookbook.com/french-mousseline-buttercream/,I use a recipe from the book The Ultimate Cake) less sweet though it is rather yellow so it is not suitable for all colours. Chestnut buttercream also tastes OK, and for decorating I prefer Korean buttercream which is also less sweet compared to the common american buttercream. There is also so called german buttercream (essentially pastry cream+butter), etc.
To prevent cake from getting soggy, you can use melted chocolate or chocolate ganache as a barrier between whatever layer you don’t want to get soggy and the cream.
Finally, the biggest problem in using stabilised whipped cream or other soft fillings in cakes in that overall the cake is too soft and can;t keep shape or support its weight well. Works fine for smaller cakes, for anything bigger or more complex shapes you need to get really creative with the structure and mixing firmer and softer parts.
