ok, so a positive but not unrealistic story with some similarities to yours.
my DP has 3 children (my step children) 50:50. One has SEND (with an EHCP for context to show it’s quite significant). His ex is extremely difficult because she’s a pathological liar and had brought a stream of different step-dads into their lives, has no boundaries whatsoever in her home and Disney parents them completely. This leaves all hard graft of parenting to my DP (and to a much lesser extent me - I tend to offer ideas but he puts them into practice). Think he does the potty training, teaching them to read, do homework and get a tutor to help them pass their GCSEs etc. He’s always bad cop but he’s completely focussed on helping them be their absolute best and adores them all.
I was a lone parent to my DS when we met. We have also have a son together. So this brings us to 5 kids total.
yes, things are busier and more chaotic when my step kids are here (inevitable with more kids), but all of us love one another and I have huge respect for my DP (soon to be DH) because he does all the hard graft of parenting for them. I cook more because I like to but he does all the laundry and plenty else as he works from home - basically we split the load.
the house tidiness levels change with 5 kids but actually I let it slip too because I’m not going to clean the bathrooms every day and so I just tend to focus on what matters most to me (kitchen) and a basic level of cleanliness. I don’t blitz when they leave, I steadily do things as normal but it is tidier because there are only 4 of us. I don’t tidy up especially for their return either (sometimes it’s pristine and sometimes it’s not) because I found living my life on an on/off switch dependent on their presence was too difficult, I live life on a constant as I have to parent 100% of the time it’s just that sometimes I’m a parent of 5 and sometimes only 2. Trying to keep a level rather than going into hyperdrive of cooking/cleaning etc. when they were here and then burning out when they left has really helped me.
I do, however, completely understand your yearning for the quiet life, I sometimes miss it just being me and my eldest. But, in truth when it is just us now it feels too quiet and I miss my step-children. So ultimately I feel like both life trajectories I could have taken (staying a lone parent or blending families) had their pros and cons and neither was right or wrong. Ultimately I love my family and I love my partner, my sons do too, so I’m happy I chose the path I did. To my mind parenting in general has its hardships and phases that strain your relationship, blending families does even more than normal, but coming out of the other side (with grown up children) will be worth the strife and I know that after all that I will still have a husband I adore and who adores me. Hopefully I will also have 5 children who still love me, but even if my step-children don’t I won’t regret being there for them.
If you’re not sure you’ll get to the other side of parenting and still have a relationship (more than housemates) with your partner I’m not sure it is worth it.