Top left line clearly sums to 6, placing [123] and [456] in the top region.
Apply r[12]c1 == r[36]c2.

The left RSL sums to 4 or 5, plaing [1234] on all its sub-segments, and putting [45] in r5c3 - colour it.

The top-right region's 6 can't go in row 1, column 5, or r[23]c6, so is in r4c6.
Row 3's 6 can only go in r2c3.
Row 1's 6 can only go in r1c1
The remaining 6s go in r6c3 and r5c4 as they can't go in r5c3.

The other [45] goes in r1c2, colour it a different colour from the RSL's [45].
r2c1 also belongs in r6c2.
The RSL's [45] can only go in r4c3 in either row 3 or in the middle elephant region.
Therefore it also goes in r3c5 and r6c1.
Column 2  and row 1 have a [45] pair, update the pencil markings.

Associate (colour) r1c5 with r2c4 and r3c3.
The [1234] quad in column 3 makes the RSL's [45] a 5.
r[23]c6 becomes a [24] pair, making r[56]c6 and r[14]c5 both [13] pairs.
r[56]c5 and r6c[45] becomes a [24] pair, but you can't make a RSL sum to 10, so r5c5 and r6c4 are both 2.
r[12]c4 becomes a [13] pair, making r3c4 a 4, resolving column 6
This also places 2s in r1c3, r2c2, and finally in r4c1.
The 5-RSL needs 3 on r2c3 and r3c1, which zigzags 1s and 3s all over the place, leaving an occasional 4.