Yeah, I've read the series too and it definitely gets better, book by book. Very much worth reading if you're a fan of military sci-fi.
I have read the whole series and liked it. He has another series that I might try and get into.
Aftershocks (July 1, 2019[8]), ISBN 978-1-5420-4355-7, 47North
Ballistic (July 21, 2020[9]), ISBN 978-1-5420-9007-0, 47North
I just ordered
Aftershocks, will report back.
And I think they made some graphic novels from the Frontline stories too?
Frontlines: Requiem (four issues, Jet City Comics, 2016)
The description reminds me a bit of Harry Turtledove's Worldwar series. (Same basic idea, but the aliens land during WW2). TBH, I didn't think it was all that good - it had a few moments, but overall it kind of dragged. But it definitely comes to mind.
Turtledove's Worldwar is rubbish. I can't for the life of me understand why so many nerds love it. Sure, it's a pretty rare premise but the series is just bad. Terrible characterisation, completely unrealistic battle descriptions, heavy handed plot and a really weird obsession with awful sex.
Frontline is a very different type. We follow just one character, not an ensemble cast, and that character goes through an actual development arc through the books. And the aliens are actually alien, like really alien, not just "humans in a rubber suit", ie aliens who behave, think and talk like humans. To be fair, that's a problem for many writers.