Have you tried a bouncy chair for naps that are harder work?
It requires you to be relentless about it. So when you know baby is tired set yourself up on the sofa with tv remote and a cuppa and bouncer at your feet. Baby in bouncer with dummy. Start a steady rhythmic bounce with your foot on the bouncer. Keep constantly reinserting dummy as needed, and be relentless about keeping the bouncing going until baby is asleep.
Depending on how over tired baby is, you might need to keep the rhthmic movement going right through going drowsey to asleep, through light sleeping into deep sleeping and only then gradually slowing down the movement. The less exhausted baby is, the easier it should be.
Also, when is best to have the last 'nap' before bedtime without it resulting in baby getting overtired?
While naps are short (ie until they are consistently 60-90m plus in length) keep them frequent and kept the cycles going all the way from when you get up until you go to bed - say 10-11pm.
So 45-60 minutes awake, nap, 45-60 mins awake, nap, repeated all day long. At any awake time that happens around 7-9pm (so awake time within these nap cycles that continue) do a bedtime routine. Bath if you need to, change to night clothes and so on. But continue having "naps" through the evening, rather than being "out to bed".
So after bath and whatnot, I'd come back down stairs and bounce back to slerp sleep in the bouncer. From then on, no more awake time but expect some wake ups. So any time baby wakes, feed and straight back in bouncer to sleep. At any wake up that hapoens around 10-11pm, take baby to bed with you and feed upstairs and into crib with you in bed.
As baby gets older, various sleep developments hapoen. One of these is naps extending. Around the time that naps extend (around 6 months usually), so the evening "nap" also extends. You start finding baby goes to sleep after the bath then doesn't wake at all until you want to go to bed. Or maybe doesn't even wake for that and you have to wake baby up to take upstairs to bed. That is the point baby is ready for a proper bedtime - so bath, nightmare, settle in crib for the night. Until then, just continue the nap-wake-feed cycles all the way through the evenings.