Then I wouldn't bother with the packages and adapters. Why not just use the breakout boards as is? Those Lattice ones are relatively small. A little less than 80mmx80mm.
As to the FPGA to choose - iCE40 are fine for this. MachXO2/XO3 would be fine too. Beware of MachXO3 though - some models have integrated Flash, others not, and I don't remember what the breakout boards include. MachXO2 are easy to use, available in QFP (if you ever want to make your own board later on), and have integrated Flash, so they don't need an external Flash chip.
As for the clock, integrated PLL's are probably more than adequate in terms of jitter - unless you were maybe thinking of building a very high-end HiFi thing. Integrated oscillators, OTOH, wouldn't be IMHO. They are just RC oscillators.
But if you are concerned, just use an external oscillator with the exact frequency you need - or a multiple, so that you can use an integrated clock divider rather than a PLL, which should give you less jitter.