I’d agree with the consensus, get someone in for this job.
Let them prep and skim for you.
There looks a couple of tricky bits there.
1 - a boarded up bit which looks below the level of the rest of the wall. That would need bringing level so probably a bit of Bonding. Even when you then skim with scrim over the join, there’s a chance it could crack a bit later on, with the different materials. There are more involved options that could make a better join.
2 - round the arch may be a little interesting if you want to use external corner beads or not.
For future -
I’m a DIYer and can generally plaster to a reasonable standard. I did a little course for a few days that taught the basics like skimming board. Note! It won’t make you an expert but will get you on the right path as a DIYer with some hands on experience. Ive since spent about 7 years renovating houses and with a LOT of help from plasterers on this forum since then. There’s a few pictures in my plastering album of stuff I’ve completed, from novice onwards.
so you can get there if you want.