How to loop variable creation and str_replace dynamically in R - r

Question

I am trying to parse multiple columns into each of their components. However the number of components varies across the columns. Specifically, suppose the following df:
id X1.startAll X2.startAll
1 ["1555726884484","1555727530298","1555727532509"]
2 ["1555735159384","1555735161545"]
3 ["1555730029709"]
4 ["1555735159384","1555735161545"]
5
6 ["1555735159384","1555735161545"]
now I have 40 of these columns (and another 120 very similar ones, to which I aim to generalize the process) and many more rows. I can do the first column quite simply using the following:
df1$X1.startAll1 <- str_replace(df1$X1.startAll, "\\[\"([0-9]+)\",*\"*([0-9]*)\"*,*\"*([0-9]*)\"*\\]", "\\1")
df1$X1.startAll2 <- str_replace(df1$X1.startAll, "\\[\"([0-9]+)\",*\"*([0-9]*)\"*,*\"*([0-9]*)\"*\\]", "\\2")
df1$X1.startAll3 <- str_replace(df1$X1.startAll, "\\[\"([0-9]+)\",*\"*([0-9]*)\"*,*\"*([0-9]*)\"*\\]", "\\3")
which yields my desired result:
id X1.startAll X1.startAll1 X1.startAll2 X1.startAll3
1 ["1555726884484","1555727530298","1555727532509"] 1555726884484 1555727530298 1555727532509
2
3 ["1555730029709"] 1555730029709
4 ["1555735159384","1555735161545"] 1555735159384 1555735161545
5
6
However, I have to do this for many columns and for many different 'array' lengths within each of these.
I have tried automating this using a for loop, however, I (1) can't figure out how to read the right number of iterations (i.e. the max the number of components in the startAll column), (2) dynamically create the variables, (3) nor how to update the string extraction dynamically ("\\i").
Any and all help on looping this process would help a lot!
Edit 2: below is a copy-pasteable sample of the data:
structure(list(X1.startAll = list(NA, NA, NA, NA, c(1555726884484,
1555727530298, 1555727532509), NA, NA, c(1555735159384, 1555735161545
), NA, NA, NA, 1555730029709, NA, NA, NA, c(1555728423843, 1555728561054,
1555728586917), c(1555725657389, 1555725657827), c(1555703810672,
1555703823206, 1555703848659), NA, NA), X2.startAll = list(NA,
NA, NA, NA, c(1555727541885, 1555727786959, 1555727897893
), NA, NA, 1555735262052, c(1555737694350, 1555737696711),
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA), X3.startAll = list(
NA, NA, NA, NA, c(1555727920770, 1555728230065, 1555728843391
), NA, NA, c(1555735331144, 1555735452321, 1555735457305),
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA), X4.startAll = list(
NA, NA, NA, NA, 1555728854666, NA, NA, 1555735589629, 1555738374484,
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA), X5.startAll = list(
NA, NA, NA, NA, c(1555728949327, 1555728988444), NA, NA,
c(1555735646258, 1555735912372, 1555735914267, 1555736071856,
1555736074184, 1555736093411, 1555736124826, 1555736238538,
1555736248889, 1555736576754, 1555736620915, 1555736874386,
1555737698921, 1555737777400, 1555737966562, 1555738152090,
1555738354075, 1555738700232, 1555738703134, 1555738716736
), 1555738415269, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA,
NA), X6.startAll = list(NA, NA, NA, NA, 1555729661240, NA,
NA, NA, 1555738960285, NA, NA, NA, NA, NA, NA, NA, NA, NA,
NA, NA), X7.startAll = list(NA, NA, NA, NA, c(1555730266934,
1555730356654, 1555730533798, 1555730535289), NA, c(1555732523945,
1555733415340, 1555733477452, 1555733748200, 1555734007271, 1555734286685,
1555734288597), NA, c(1555739871726, 1555740315324, 1555740328252,
1555740329835, 1555740538272, 1555741140561, 1555741143555, 1555741152932
), c(1555743562826, 1555743566386, 1555743593201), NA, NA, NA,
c(1555727969354, 1555727985539, 1555728064237, 1555738166838,
1555826735910), NA, NA, NA, NA, NA, NA), X8.startAll = list(
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA,
NA, NA, NA, NA, NA)), row.names = c(NA, -20L), class = "data.frame")