I hear you OP. Totally understand your anxiety. I made my peace with MIL a decade ago now, but it took a few years for me to have the confidence to set - and assert(!) - my boundaries.
I shan't go into the details but we had such awful visits, I used to dread her coming. Her last newborn visit was so bad, I finally put my foot down and vetoed all future overnight visits until I felt ready to care for both my DC and MIL. Which took a year or two.
So speaking from my own experience, my advice would be to pick your battles.
I would not fight the gifts (they will not overshadow mum and dad). Express your gratitude and then allow yourself to manage them as you see fit. Put aside what doesn't work for your family or isn't age appropriate. Space them out. To avoid emotional arguments at gift giving time, have any necessary talks about gifts in between visits and be prepared to patiently repeat them.
Remember, you and DH are the ultimate authority when it comes to setting rules for your kids. And then embrace the idea that grandparents are allowed to spoil their grandchildren, occasional rule breaking included. If it gets too much, again communicate your wishes in between visits - in a friendly but firm manner. Be patient and persistent.
As for your immediate visit, I recommend feeding for as much and as long as possible. DC3 did not want to be held by MIL, screaming whenever she tried. I realised that her perfume was too overpowering for the sensitive nose of a newborn, but couldn't tell MIL without risk of offending her. So nursing sessions were twice as long as normal and if I decided to get up, once asleep, baby was put down in the cot. I leaned on DH a lot - having him look after his DM, so I didn't have to.
I also echo what PP have recommended - talk to your DH before the visit. Be open about your feelings and your boundaries. Enlist his help in asserting them.
Think about what would help you most before your PIL arrive, so if either of them asks what they can do to help you can pick from a whole range of things as suitable to the situation and as needed by you.
Practice saying no. I'm not kidding. With a smile and conviction, because it is your right to do so. And it is right to have and set and assert boundaries that serve you and your DC.
And always always repeat to yourself the mantra that this, too, shall pass. It can serve you well in moments of parenting nightmares. It was sometimes my first and last thought of the day and it was true and I took comfort from knowing that. (It may reduce your stress levels by just one percent, but that is one percent not burdening you anymore. I had days where that one percent really did make the difference.)
I did, back then always and with great longing, reflect on my DM who you would absolutely want around as she very carefully navigated those first few months of my motherhood. Not interfering, but willing and ready to advise if that was wanted. Which included her watching us make some annoying first-time-parent mistakes, but she held her tongue all the same. And since those first months almost 23 years ago, she has been my rock. Who sometimes told me me some hard truths I needed to hear about my parenting, but never forced them on me. And who helped in the house so I could have a rest.
So if you do have a DM or DF like that, maybe you can also bear your PIL's visit in happy anticipation of their visit. (Or anyone else really, whose visit you do look forward to.)