9s along the edge always terminate their thermo immediately with sum 9.
=> r9c5 is 9

c1's down thermo doesn't sum to 9, r2c9 must be c9's 9.
=> r1c9 is 3 to make the c1 down thermo sum to 12
=> r1c8 is 6 to satisfy r1 and c8's thermos
=> r1c4 must be r1's 9
An 8 on an edge can only go up to sum 17, or terminate.
=> r1c1, r4c9, and r9c6 are the only available places for 8s.

A 7 on an edge could go to 16, or 15, and then maybe 24
=> r1c3 and r9c1 are the only available places of 7s on their edges

A 2 sum outside an edge can only be a 2 outside a 1
=> r9c3 is a 2, r8c3 is a 1.

Column 2's 14 sum can only be 5-9
=> r8c1 is 4 to complete the thermo summing to 13

A 6 sum outside an edge can be 1-5-low, 2-4-low, or 6-low.
=> r7c1 must be 6 with a low number to its right
=> r7c2 is 3 so the row 7 thermo stops
=> r7c3 is 8
=> the col 2 thermo down can't start 1 or 6 so must be 2-4-1
=> the row 2 thermo must begin 3-4-...
=> column 1's 9 is in row 3

A 5 sum outside an edge can be 1-4-low, 2-3-1, or 5-low
=> r1c6 can't be 1 or 2, so is 5

column 1's 5 can't go in rows 2, 5, or 6, so is in row 4.
column 3's 9 can't go in row 2, 5, or 6, so is on row 4
=> r4c2 is 6

row 8's thermo summing to 28 must stop at the 9
=> r9c4 is 3
=> the remaining digits are 6-4-1

The thermo summing to 18 in row 8 must contain box 9's 8, but that cannot be in column 9 or extend column 7's upward thermo
=> r8c8 is 8
=> r8c9 is 5
=> r7c8 must be the 9 that terminates a 7-9 thermo on row 7, rather than the only other possibility which was a 2-5-9 thermo.
=> 
=> r7c9 can be pencilled [12]

column 4's upward thermo can only complete as a 3-7
=> sudoku in row 8 puts a 3 in r8c7
=> r7c7 is 2
=> column 7's downward thermo can't be 4-5-low, so is 1-8-low
=> row 1 completes with a 4 in r1c5 

row 6's thermo summing to 6 cannot be 1-5-low or 2-4-low, it must be 6-low

row 3's thermo summing to 16 cannot contain 4 terms, as the smallest three available terms already sum to at least 14
=> that thermo is 4-5-7
=> 5 and 6 resolve in box 1
=> r5c9 is 2
=> r5c1 is 1
=> r5c2 is 8 completing the thermo
=> the thermo in row 6 is 2-7

The 3-4-5- thermo row 2 can only extend -6-7 to achieve the required sum of 25
=> The column 5 downward thermo can only sum to 19 by ending with an 8

Technically, at this point, you can just sudoku it home - the thermo summing to 18 wasn't used as a positive clue - however, had it been absent, you could not have drawn the same conclusions about whether other thermo sums were forced, so it wasn't redundant.

2 and 3 placeable in row 3
Naked 8 in r6c4
Naked 4s in r7c6, r4c7, r5c4, r6c3
1 and 5 placeable in column 4 and row 7
7, 3, and 1 placeable in box 6
2 and 7 placeable in row 4,
resolving 2 and 6 in row 8,
and everything else just slots in.