I think some respondents have been wilfully obtuse and unkind in their reading of your post. I think sub "relentless" for "thankless" and perhaps not posting in AIBU and you'd have got different replies. I didn't read it as you're expecting actual thanks from your child, more that you feel on a basic level parenting can be a grind.
I have seen many threads on MN where women say they feel worn down or overwhelmed by the day-in day-out intensity and drudgery of caring for small children and the peripheral house / life stuff that comes with it and get a lot of empathy. How many people comment saying they could never be a SAHM or were climbing the walls by the end of Mat leave because they found being at home with babies / toddlers isolating and dull?
My own experience so far of parenting a 3 and a 1 year old (the latter yet to sleep through which definitely colours my outlook) is that is mostly drudge but made worthwhile for those perfect moments whether it be seeing your child learn something new or just giving you a sloppy kiss. BUT on a day to day level I sometimes find it very wearing. Sometimes how I feel can change in the space of an hour when one moment it can feel the most wondrous, life-affirming experience then rapidly spiral into inane, repetitive and frustrating.
Today oldest DC declared "lovely dinner mummy" while heartily tucking in - this filled me with joy, but then a second later youngest DC hurled her untouched plate across the kitchen spraying gravy and broccoli lumps with impressive range - I could've wept. I'm a a SAHM and sometimes feel I spend more of my life on my knees picking up food off the bastarding kitchen floor than I do anything else. It's this sort of stuff - it's relentless!
But back to your post - it depends, do you feel the love for your DC outweighs the "thanklessness"? If not, then a second child probably isn't for you. I figure that a lot of the weariness I feel now will pass as they get past the stage of needing me less for the more basic aspects of self care. Also getting back to work, having lots of solid night's sleep, a social life that doesn't happen at a toddler group would all help....Oldest DC can take themselves to the loo, help with small tasks and have some genuinely interesting conversations now so I see some hope for the future.