@SometimesIFeedTheSparrows - my cat (and his brother, RIP) is long-haired with a really dense coat. Spot-on treatments were always a nightmare. Firstly, because he hates it and is a wriggly fucker. Secondly, I always worried that is wasn’t getting the solution directly into the skin, no matter how hard I tried.
Game changer for me was a flea injection which is given twice a year (one which coincides with annual check-up appointment). Still leaves worming every 3 months, but I can cope with that (plus, he’s not a hunter cat so slightly less worry for me).
Might be worth seeing if your vets do the injection. It’s called Program. I do the health plan at my vets - which spreads the cost of flea/worming/booster health checks over 12 months (and works out cheaper than paying for each treatment individually). The plan covers injections as well as spot-on flea treatment.
Result in this household - much happier cat and less stressed slave!